Engine stalling off of acceleration

[Mac-Porsche]

Sorry for posting yet again on this topic!... Been trying to tackle these CIS issues since finishing a full rebuild for past 2-3 months... Losing it on this...

1981 911sc - US model
What I did during rebuild...
- Split crankcase - had oil pump rebuilt, sent all various components to machine shops
- Installed 97mm P&C set, 10:1 CR
- reground camshafts to 964 spec
- ignition distributor (including vacuum advance) rebuilt
- fuel distributor rebuilt
- fuel injectors cleaned and inspected
- new airbox with pop-off valve (previous one discovered cracked/blown on bottom)
- new intake boot
- SSI heat exchangers installed
- New oxygen sensor

Engine fires up great since rebuild. No oil leaks. Fires right up every time, hot or cold. Revs really nicely.

However... idling has been a problem since finishing rebuild, surging idle... Bought fuel pressure gauge, checked all fuel pressures (system, control, etc) - all checked good. System pressure was showing 65psi (~4.5 - 4.7 bar), so I put in a brand new fuel pump since it was on the low side (for good measure). At this time, I thought maybe it was bad fuel. Emptied gas tank completely, blew down all fuel lines, replaced fuel filter, put brand new fuel in. Also replaced spark plugs (old ones were black and oily - thought maybe they were fouled). Surging idle still persisted...

Around this time, it was running so rich that it was sputtering raw fuel. Did the "baseline" calibration, where I pulled a fuel injector and leaned until no fuel from injectors when pump is on. Got it running/idling much better, but my wideband O2 sensor (Innovate LC-2) meter showed it was running very lean (16-18 afr) when warm.

Frustrated, I finally took it to a well respected local Porsche mechanic. Hooked it up to smoke machine to check for vacuum leaks - no leaks. Then hooked it up to gas analyzer - showing ~5.5 CO%, which I was told was way too high. Reduced the CO% down to ~3.0 with mixture screw. Also adjusted the idle speed to 950rpm and double-checked timing - all good. Afterwards, 'seemed' to idle better, but I did notice some slight surging before I left.

Immediately after leaving shop, car stalled twice on the way home. Which had never happened before. Both times were from letting off gas after acceleration/high revs. Letting off the throttle immediately would cause the car to drop to very low idle (~400 rpm) for a couple seconds before picking back up to where it needs to be. Sometimes, it would drop as low as 200rpm and stall (as what happened twice on the way home).

Read on these forums that it's a sign of running too rich. Pulled the fuel injector and ran fuel pump, looked good. Hooked up Innovate LC-2 sensor which still showed 16-18 afr at warm idle - so that adds more confusion...(starting to not trust that meter)...

Not sure what to do anymore... I feel like I've literally tried everything... Is there any other good gas analyzers or AFR meters out there? I did notice that the Porsche mechanic changed the fuel mixture without the car warming to temp and without disconnecting the oxygen sensor. Plus he set it to ~3.0 CO% as mentioned before - where the Bentley manual suggest to set the '80 - '83 models to 0.4 - 0.8 CO% with oxygen sensor disconnected. Could this be my problem? If so, shouldn't the lambda system correct that when hooked up?

[Glenfield]

Hoping AndrewCologne pops up for this discussion.
Based on your symptoms, it seems like AFR reading may be too high as it doesn’t correlate with the CO measures. CO measure also sound like they’re set too high with oxygen sensor unplugged (range you mentioned sounds right). Have you tried the method of keeping O2 sensor in and testing duty cycle at warm idle? Should be around 50 for Lambda.

This is purely theoretical for me as am about to go through this. I’d look at AndrewColognes posts or check out his long post in his signature.

[Black 993]

If you're actually at 16:1 AFR at warm idle, you are too lean. Adjusting the CO on CIS is super easy, a 3mm allen between the fuel distributor and sensor plate. Can't help with whether you need to plug/unplug the o2 sensor as my CIS 911's were from the 70's. But it sounds like you just need to set your CO correctly.

[Walt Fricke]

Mac - my understanding is that the Lambda doesn't affect idle, only part throttle/cruise. Idle uses a default value for the frequency valve. Disconnecting the wide band sensor would guarantee this, however.

What you mention about what happened at the shop sounds wrong to me also.

Check for Gunson - I think they made a CO meter. Those may come up used now and then.

There is a much fancier older generation electronic system which was cutting edge in shops back when, check HC and CO both (don't know about NOX). A mechanic lent his to me once, but don't recall the brand. It was about the size of a super super thick laptop, not the huge machines on a cart with casters which lots of shops had back then. In my state lots of shops could do the required emissions tests, but then the state created dedicated test centers - those became available used, too, but you need space in your garage if you get one.

Since you have an A/F meter, why not try adjusting the idle (iterative process between the mixture screw and the idle bleed screw) to get a better warm idle - like keep nudging it until you get to 14.7 for want of a better number?, with the idle RPM adjustments to about what Porsche says it should be. Does it then surge? How does it then drive? You can adjust with the engine running (at idle) watching your meter if you can see it from back there (I can see mine).

If you have made sort of large changes in A/F, might be a good idea after driving a bit to do another plug check. After idling should give you an idea of how idle is doing. For actual driving the protocol (fairly easy at the track) is to drive at a pretty high RPM/load, then turn off the motor, clutch in, and coast (brakes OK here) to a stop. Then pull a plug and look at it. It won't have had time to change from what wide open throttle does to it, is the theory there. Might be hard to do using your garage on a residential street.

Spark advance also affects idle, but should be easy to set to old specs? You've changed your engine some, might that change what the advance does? Wouldn't think it is behind the larger issues, though.

[targa72e]

This might help. You can figure out CO with your wide band O2 meter. The chart below shows the relationship. your 16 to 1 should be between .5 and 1 CO.



If you don't disconnect the O2 sensor correction it will try and drive the A/F mixture to 14.7 as you try adjust the mixture.

John

[Walt Fricke]

John - so what about the literature, Bosch and DIY guys like engineer Jim Williams, who tested all the combinations for the nice chart he made, which says/shows/suggests that at idle, the three wire switch on the throttle disables the O2 control of the FV, and instead uses a set PWM signal of a given duty cycle to the FV. Just as, at WOT, it does the same thing, but richer.

Is this wrong? If I were to reconnect a narrow band sensor, and hook up a dwell meter (though I would just use a laptop digital scope, hooked by a BNC to a port I installed on the acceleration enrichment smaller box) I could see the pulse width modulation in action, as the FV drove the idle mixture rich, and then lean, etc to arrive at 14.7? That is what I would expect at cruise (1/3 to 2/3ds throttle) with the sensor. But not at either extreme. What am I missing here?

My acceleration enrichment box has three home brew additions. One is the BNC to read the signal to the relay, which sends it on to the FV, and is a nice PWM square wave, which is what you see at the test port back in the engine compartment, where you hook up the dwell meter. The other BNC connects to the point inside the large ECU box (pin 15) where another more complex wave form appears, later to be converted to the square wave. That signal isn't all that useful, at least if the PWM signal is good.

Finally I inserted an external LED which comes on if the unit is getting power. I went a long time wondering why things weren't working right, only to find the fuse for all this, which also serves the interior dome lights, had blown. My roll cage precluded the dome lights, I cut the wire to them, but didn't tape its end well enough. So I can just look at the LED to know there is power to the FV system.

I have also moved the relay (standard plug in relay, basically) up off the floor. With cool suit boxes you get water on the floor pan. It had gotten into the relay and dissolved the relay's spring which holds the moving contact in its normal position. Took a while to figure that out.

But I digress - does the O2 sensor modify the FV pulse from its default idle and WOT set pulse widths/duty cycles in one or both of those modes? The three wire switch opens (or closes) the ground first to an idle connection in the ECU then to both, then to the WOT circuit via the aux unit in the 81-3s (directly to the ECU, I suppose, in the '80 which doesn't have this accel separate unit).










The little stand alone scope pictures are from the port in the engine compartment. The PWM train pictures from the better scope using the BNC port. The base signal annotated was done by someone else (who knew what the trace meant, though I could capture the same thing).

I also measurerd the overall PWM frequency at 69-70 Hz, same as others have. About the same as a light bulb. The transition from closed loop to open loop, with the O2 sensor working, is probably near instantaneous (maybe some spiking).

Scopes and pulses are one thing, getting the WOT AFRs I think I want is another.

[Walt Fricke]

I just noticed that the better scope shows a ~36% duty cycle. Inverted, that would be close enough to 65%. Makes the scope work with less conversion than an analog dwell meter. It will record for a while, too - could use it if it proves useful for my racing purposes to hook up the stock O2 sensor.

Noting that the ECU has a separate pin for the O2 sensor input, I wonder if I could set up an oscillator or something, triggered by the WOT switch, which would cause the FV to operate at a 55% duty cycle in that range. A faux above 0.5V/below 0.5V signal at the right frequency. Would need to be PWM, not just frequency? Or just a frequency, since the equation is above a voltage, go one way, below it go another? With a variable frequency test input, what was needed for 55% could be determined by cut and try if not calculation from really understanding what is going on? Easier than trying to figure out where in that battleship of a PCB Bosch creates the default values, and how to alter something on the board to do that? I think it is all analog, with no tables of values or the like.

[ahh911]

Isn't the deceleration valve supposed to help out in the case where it's starved of air? I imagine that if all the leaks are plugged up on the intake manifold, injectors, runners etc... after a rebuild making the car more sensitive to throttle plate air regulation as there is less air coming from the leaks, hence the decel valve's increased importance when quickly releasing the throttle? At high vacuum it's supposed to give a shot of air to prevent stalling, at least I read that somewhere, maybe Jimsbasementworkshop, maybe online. Also, doesn't the accumulator show similar symptoms if not working correctly? Excuse me if this is nonsense, but I'm pretty sure I've read that too.

Phil

[Walt Fricke]

AAH - interesting speculation about the accumulator. Typically this tends to be discussed over hot start problems? But one of its roles supposedly is to smooth out variations in pressure from the pump. I can't offhand visualize how those would happen - it is a roller or vane pump, which sounds like its output pressure should be pretty steady. But typically what we know is system pressure, which is past the accumulator and the FD pressure regulator - and I've not seen that fluctuate.

[targa72e]

HI Walt,
My comment was more in general to say that you do not have to have a CO meter if you have a wide band and if something electrical is controlling mixture based on O2 sensor feedback then trying to adjust mechanical system to change mixture will be compensated for by feedback control system. All my CIS cars have been earlier models and did not have a frequency valve so I have never looked at how and when they work to control mixture.
I sounds like you are looking to see if you could modify the signal to FV to control mixture for tuning purposes vs emission control. It would be interesting to see if the FV amount of control can make a significant change to AF mixture at wide open throttle or if the amount of control it has is too small and only works well to trim mixture when fueling requirements are low (idle and/or part throttle).

john

[Walt Fricke]

For a Pavlovian response to a CIS question about a different issue than one I am working on which isn't going to be helpfur for Mac and his problem.

Plus after reviewing things, I find I had the wrong idea in my head. On a hot (35*C) engine, the FV/O2 system DOES operate to adjust the idle AFR, just as it does for cruise. Which would be why the manuals say disconnect the O2 sensor when setting the idle AFR.

Mea culpa.

[Dave Kost]

Im not convinced that your lambda system it working even though you said the dome light is on.

Do this test.

Get the car running and then disconnect the OX relay under the passenger seat. What happens? If you are using the stock USA CIS system from a 1981 911 the Frequency Valve MUST be operating. If the FV isn't working, you can get to run butt not run good by richening the mixture.- CO 5%!

If the FV valve is not working, pull the connector and put 12v (Fused) to one terminal and a ground to the other. Key on, push up AFM arm and the FV should vibrate.

Also, what is your cold control pressure?

Where are you measuring CO? Without a cat, your CO should be 1.0 to 3.0 %- you have SSI right.?
Make sure you disconnect the O2 sensor when setting the CO if the FV is working.

Let us know.

[Walt Fricke]

Another thing you can do is check the relay which is on the left side rear of the passenger seat. It is an aluminum cube, plugs into a plastic socket, and is clipped maybe onto the seat rail. Not the larger rectangular silver box I mistakenly posted a picture of. Slide the passener seat all the way forward (or back) so you can see the big "ECU" box.

When you turn the key to run, this relay should activate. You might be able to hear the click it makes, and should, if you have your fingers on it when you turn the key on, feel it click. If it doesn't, you may not be getting power to any of the electronics (other than the CSV and WUR and maybe AAV) back there. The relay can go bad. Of course, you can take your wiring diagram, see what wires go to the relay, what they do, and with a volt meter easily check to see if you are getting 12V to he relay, and 12V out of it with the key on.

When I ran my -82 engine without any of the electronic stuff in the car, it was hard to start. Hooking up the electronics even without an oxygen sensor cured that right away. Which is why guys are stressing being sure the FV is working.

[Mac-Porsche]

Thanks for all the replies!! I didn't realize this post was gaining so much traction.

So I found the solution, and it's a lot more embarrassing than all the suggestions put forth... Turns out the ignition wire to cylinder #2 was not connected properly. I reconnected it and simultaneously swapped the Lambda ECU with a one from a '83 911sc. Car idles/runs great! Hooked up the afr meter at WOT and it shows a perfectly flat 14.7 when idling.

Put ~200 miles on the car since rebuild, really pushing it at times for proper break-in and to ensure the rings seat. Just did a compression test today and all cylinders show 175 - 185psi, (installed 10:1 P&C set with 964 cams). So I think I'm good on that front.

However, now I'm running into something else odd... When starting the engine cold, it takes a bit for the engine to start (turns over 3-4 times prior to starting). After that, it'll start up immediately in each consecutive start. I also noticed that the first ~5 minutes of driving when cold, it is reaaallly sluggish in accelerating. Plus, it will drop to ~500rpm when letting off the acceleration... From what I've heard, it's acting very rich when cold (smells rich too).

I disconnected the O2 sensor and got the car to WOT, it shows a consistent ~13.0 afr. Got a Gunson gas tester and it shows 3.0 - 3.2 CO %. Which is more rich than the manuals suggest. We don't have emissions testing here, and I'm not concerned about fuel efficiency, so I figured that's right where I want it.

Is the sluggish to start up and accelerate when cold a normal behavior? Once fully warm, it drives and accelerates great.

[Walt Fricke]

What do you mean - hooked up the AFR meter at WOT, got the car to WOT (wide open throttle)? This is driving it on the road? Or reving it in the garage to 5,000 rpm with no load? This is confusing. Remember, you set the idle AFR or the CO with the little hex screw with the O2 sensor disconnected.

However, you should not have cold start problems. I'd look at the thermotime valve, and the cold start valve. If the TTV isn't working, the CSV won't do its thing. That might have something to do with the starting problem. The CSV only squirts its extra fuel for a very short while.

The slow warm up/sluggish driving could be something else. There are two temperature sensors, one on the right chain housing cover, and one in the engine vent cover (harder to get to). They tell the ECU whether your engine/oil is over X degrees, and then over X+Y, for warm and then hot running. I am dubious that those sensors fail - just a resistance element with no moving parts - but worth checking, especially to see if the wiring is right.

Then there is the WUR. To check that you need the control pressure measuring gauge tool. Control pressure, as the engine warms up, is supposed to increase (which means the fuel distributor leans out the fuel delivery). As is easy to see looking at a diagram of the WUR, a spring strip depresses the WUR's main spring (which pushes up against a diaphragm valve, and controls how much control pressure is bled off) when cold, reducing the effective main spring pressure and richening the mixture. As the spring heats up (it has its own electric heater) it reduces its pressure on the spring, and when fully warm it is simply out of the way.

Which is why you first check cold control pressure. Then warm. You want both of them to be within the pressure specification band.

In addition to the gauge setup for checking control pressures, and an ohmeter for checking the TTS and circuitry generally, you should download and print from the factory manual the two pages of the wiring diagram which show the ecu/lambda etc system.

That's pretty nifty that you have a Gunson as well as an AFR meter. Somewhere out there there is a conversion table: If CO is X, AFR is Y. Lets you double check things.

[Walt Fricke]

Plus - is the frequency valve buzzing/clicking palpably? It should do that at idle as well as at any other RPM, so it is easy to check with your hand.

Rereading, I see you have the pressure gauge for checking control pressure. At least I think you do - you just called it a fuel pressure gauge. If it connects in line between the FD and the WUR, and has a valve, that's the pressure gauge needed. List your cold and hot control pressures, as well as your system pressure. Those are the figures the CIS gurus always ask for, and with good reason.

[Mac-Porsche]

Haha - my bad! Read "WOT" in other forums, and without knowing any better, just assumed it stood for "Warm-Operating-Temperature"... I can see why that would be confusing to read. I really meant to just say it was showing ~13.0 afr when idling warm (after ~20 minutes of driving).

Will follow some of the suggestions you mentioned. I do have a pressure gauge. I checked the pressures before and they seemed within spec - but I feel as though this issue has only developed in the last week or two. So maybe something has changed. Will keep posted. Thanks!

[Mac-Porsche]

Checked the fuel pressures. Still same as when I checked them ~1.5 months ago.

System pressure (fuel pump on, engine off) - 65psi (4.5 bar)
Cold control pressure - ~25psi (~1.8 bar)
Warm control pressure - 50psi (3.4 bar)

When I started the engine, it started at 1.8-2.0 bar and just slowly rose from there to 3.4 bar over the course of 30 - 60 seconds. Then just sat right at 3.4 bars from then on.

Engine still hesitates to start when cold... Had an embarrassing moment with a friend who saw the car for the first time after rebuild and wanted to hear it start... Took almost 10-15 seconds and pumping the gas to get it started... He just looked at me blankly and said it sounds like it needs more tweaking...

I just ordered a new O2 relay (they're inexpensive, so I figured worth a shot). Will also start checking TTV and CSV with ohmmeter.

Oh! and I also discovered when showing my friend the car that I forgot the gas cap at gas station when last refilling (1-2 weeks ago)... sucks... highly doubt it, but would that affect cold starting?

[Walt Fricke]

"System pressure (fuel pump on, engine off) - 65psi (4.5 bar) BOTTOM OF SPEC, BUT STILL IN
Cold control pressure - ~25psi (~1.8 bar) SEEMS RIGHT DEPENDING ON AIR TEMP
Warm control pressure - 50psi (3.4 bar) SEEMS RIGHT

When I started the engine, it started at 1.8-2.0 bar and just slowly rose from there to 3.4 bar over the course of 30 - 60 seconds. Then just sat right at 3.4 bars from then on."

This is just what the WUR is supposed to do - low control pressure for rich, easy cold starting. Then leaning it out (rising CP) until the heating coil in the WUR has caused the bimetalic spring to completely disengage from its job of pushing down on the pressure springs.

I'd be inclined to look elsewhere for your issues. What are your AFRs and CO%s?

And why are you pumping the gas? There is no acceleration pump feature on these cars - pumping doesn't really do any good, and may help flood it. Yes, if it coughs and sort of catches, a bit more throttle probably helps. But that indicates it is out of tune - as you know.