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Cruise surge/boost issues

About 9hrs after signing the paper work on the car it developed and issue with cutting out under boost occasionally, down on power, stuggling to rev etc. I had John do a full tune to get us to a good baseline.

So post tune up (plugs, cap/rotor, valve adjustment) I took the car for a jaunt on some back roads late in the evening. Car pulled hard and clean, no more cutouts under boost, no more feeling like it struggles to build rpm. Sweet!

There is now some lifter tic on the passenger side bank but I don’t have any point of reference as to what is normal valve noise, nothing alarming however. John said most the intake valves were way too tight and one cylinder needed the exhaust valve tightened. The car does feel like it may be just a hair down on power but I am fine with it given it makes power smoothly and cleanly.

A few days later I look the car for a longer cruise and noticed that once the car is warm and you are holding steady at 2100-2600rpm the car surges. It feel like someone is just waving their foot over the brake. Not a misfire, but a momentary hesitation, the tach doesn’t register any drop in rpm but your head just bobs forward and back. Get on the power, no issue. Lift completely, no issue. Hold it at a steady rpm, head bobbing. Argh. Over the next week or so much more road testing. Issue is consistent and ever present once the car is warmed up. Around town you don’t notice since you are always accelerating and decelerating, on the highway however it is very noticeable and after about 10mins super aggravating. Called John and setup an appt.

The car when I bought it had the 02 sensor disconnected, post tune up it was left unplugged. I have heard from a few different folks that this is not all that uncommon (true)? John plugged it in to troubleshoot and found it was completely dead anyway. He tossed a used unit on and dialed in the cold and warm mixtures. After a road test the car was ready and I picked it up. Unfortunately I only made it about 10 blocks. The car had trouble accelerating in any gear and was really having trouble revving. Swung back to shop and we disconnected the 02 sensor and reset all the mixtures again. Car went back to its normal self, phew. I took it out later that night, the surging remained. Bummer. John did relate to me that all 930s exhibited some surge at cruise, and I thought perhaps the cams could be exasperating the issue. I agreed to put some more miles on it and see what happened. I put another 500 miles on the car and no change. Car was great in every scenario except cruising.

Skip to Friday, the car has been great just as long you never want to keep it at one speed. I popped the engine lid when I got from home to work to take a quick peak. I like to keep an eye on “new to me” engines and honestly I am kind of at a point where I think it’s time to take a little more mechanical ownership. One of the vac lines looked janky to me, the line running to what I believe is the advance side of the distributor vacuum canister. I reached in to snug it up on the nipple and low and behold, this is what I found.


IMG_5678.jpg by thecrashingdoor, on Flickr


IMG_5680.jpg by thecrashingdoor, on Flickr

Yep, a giant hole. The nipple was basically poking clean through. I cut the line back and put it back on the nipple with a ziptie to secure it. Drove the car and…..all the initial pre-tune up issues are right back. Cut out and struggling under boost arghh. The surging remains but feels slightly less severe. So here we are, back at square one.

So here is my new theory. The root cause of the boost cutouts, surging and struggling to build power are all ignition related. When I think of the problems in terms of advance and retard it begins to make sense. The cruise surge could certainly be someone cranking advance and alternately yanking it back out. The struggling to make power/build rpm under boost could also certainly be excessive retard. I have been researching the 930 ignition system and the vacuum diaphragm/distributor while not a common problem could be suspect. Old grease can cause the weights in the distributor to mess with timing. Vacuum diaphragm can deteriorate and mess with timing etc.

If I disconnect the line from the distributor to test is there any danger? Essentially I think this is how the car has been running with the giant hole in the line.

I have a new front fuel pump, new rebuilt WUR and a adjustable WUR on the way. I am going to swap the pump just as preventative maintenance since its a little noisy. Its going to be awhile till I can get the WB 02 installed still.

Any suggestions folks?
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Old 02-17-2014, 12:10 PM
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16Volt, do you have your smog equipment deleted? If so are you vac lines routed like the below diagram link?

'78 SC Turbo Conversion Begins
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Old 02-17-2014, 12:36 PM
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At some point someone did delete the smog equipment from my car.

re: the drawing

The black line from the left side of the distributor vacuum puck runs to the underside of the throttle body facing the rear of the car. Something to note, where it attaches to the TB there is one of the old one-time use clamps so its doubtful if it has ever been connected anywhere else.

The red line from the right side of the vacuum diaphragm appears to run back to the crank case breather box.
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Old 02-17-2014, 01:43 PM
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I sent you a PM 16Volt, too many words to type.
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Old 02-17-2014, 02:20 PM
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Regarding your mechanic's comment that all 930s surge somewhat at steady cruise - he's mistaken about that. Is that John Walker you are referring to? If so, I'm surprised to read that.

The first thing you need to do is replace all of the small vacuum hoses as they are, no doubt, all in the same shape as the ones in your picture. Along with that, you need to try and check for any other vacuum leaks as those will upset the the K-Jet system in a big way (such as - vacuum leak causing a false rich condition which can then result in missing/bucking/hesitation).

Do you have an air-to-fuel ratio meter on your car? It would be interesting to see what your AFR is at steady cruise - too rich and too lean can both cause the running situation that you describe.

Also, insufficient charging voltage can be a cause (but probably wouldn't show at just steady cruise, though) - can you take voltage readings at different RPMs?

Also bad electrical grounds - I had a bad ground (ground for the engine wiring harness - bolts to the coil bracket) that made the Lambda system erratic, and I chased my tail with that for a couple of months until I found it!

Another thing worth mentioning - with the Lambda assist, K-Jet (like you have), you can't just unplug the O2 sensor, set idle CO2/AFR, and expect it to run right at steady cruise as it will now likely be too rich at that engine speed without the Lambda computer raising control pressure. You would need an onboard AFR meter to check this. If it is then too rich, then you would need an adjustable WUR so that you could tune for a good cruise AFR of 14.5 - 14.7. Also, running the RPM up, at a stand still, with a tailpipe sniffer in place doesn't do any good because there is no load on the engine. I'm not saying that any of this has been done to your car, but thought it might be something to consider since you made mention of the disconnected O2 sensor.

Last edited by Ronnie's.930; 02-17-2014 at 05:21 PM..
Old 02-17-2014, 04:10 PM
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I would tend to agree with Ronnie, you're mixture (AFR) is off a little bit. I had the same issue years ago following the purchase of my 930...steady state cruise at around 2000-2500 and an ever so soft surging. Drove me batty, adjusted the mix, and all was well.

I also pulled the plug on the O2 sensor very early on and didn't really have any issue with dialing in the AFR's I wanted. Remember the O2 sensor only does it's thing primarily at idle and lower rpms. Unplugging it and running open loop sets the frequency valve's duty cycle at something like 50% so its affect becomes a constant rather than a variable. Unless your locale requires emissions testing, I would take that Lambda sensor out of the loop....just one less thing to tune around. I used an old school Gunnson GasTester to set the AFR at idle by adjusting the mixture control screw on the fuel disturbutor. Then later upgraded to an on-board Innovate AFR gauge. The Gunnson actually worked pretty well but recently it got a little hot and kinda....like....melted.

My $.02 worth.......
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Old 02-17-2014, 05:33 PM
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Did you take the ignition leads off at any time post problemo?

Sure replaced n correct order?
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Old 02-18-2014, 01:07 AM
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Quote:
Originally Posted by Ronnie's.930 View Post
Regarding your mechanic's comment that all 930s surge somewhat at steady cruise - he's mistaken about that. Is that John Walker you are referring to? If so, I'm surprised to read that.
Its entirely possible I miss-heard him. Not a big deal, at the end of the day the surge is an issue and our first attempt to resolve it was purely focused on mixture.

Quote:
Originally Posted by Ronnie's.930 View Post
The first thing you need to do is replace all of the small vacuum hoses as they are, no doubt, all in the same shape as the ones in your picture. Along with that, you need to try and check for any other vacuum leaks as those will upset the the K-Jet system in a big way (such as - vacuum leak causing a false rich condition which can then result in missing/bucking/hesitation).
Yeah, I am decently versed in CIS on other vehicles. I have the car torn down to the intake manifold at this point. You can see in this pic where the black line from the distributor vacuum canister goes into the throttle body.

IMG_5719 by thecrashingdoor, on Flickr

The red line from the other side continues back to the crankcase breather. Is there anything special about the red hose material wise? All my replacement hose is black. All the other lines do in fact look ok, I am hesitant to go in and cut the one time use clamps off since I know what a pain that is. Keep in mind the motor is in the car.

Quote:
Originally Posted by Ronnie's.930 View Post
Do you have an air-to-fuel ratio meter on your car? It would be interesting to see what your AFR is at steady cruise - too rich and too lean can both cause the running situation that you describe.
I do not have the WB 02 installed in the car unfortunately. It will probably be mid march before I can get the time/help to get all that stuff wired in (I have a PLX unit).

Quote:
Originally Posted by Ronnie's.930 View Post
Also, insufficient charging voltage can be a cause (but probably wouldn't show at just steady cruise, though) - can you take voltage readings at different RPMs?

Also bad electrical grounds - I had a bad ground (ground for the engine wiring harness - bolts to the coil bracket) that made the Lambda system erratic, and I chased my tail with that for a couple of months until I found it!
I read your thread on that, what a nightmare. The grounds look great but I will go over them again just to be sure.

Quote:
Originally Posted by Ronnie's.930 View Post
Another thing worth mentioning - with the Lambda assist, K-Jet (like you have), you can't just unplug the O2 sensor, set idle CO2/AFR, and expect it to run right at steady cruise as it will now likely be too rich at that engine speed without the Lambda computer raising control pressure. You would need an onboard AFR meter to check this. If it is then too rich, then you would need an adjustable WUR so that you could tune for a good cruise AFR of 14.5 - 14.7. Also, running the RPM up, at a stand still, with a tailpipe sniffer in place doesn't do any good because there is no load on the engine. I'm not saying that any of this has been done to your car, but thought it might be something to consider since you made mention of the disconnected O2 sensor.
You're correct and I have read as much in various threads. I have also read the counter threads like Mark relates that its possible to run open loop as with good results as well. I guess the real test here would be to pony up for a new stock 02 sensor and test.

The ignition leads were likely removed as a part of the tune up but given how great it ran before I fixed the vacuum line I am pretty confident they are fine.

OilOnly - I got your PM and will give you a ring tonight when I am in front of the motor.

Does anyone have any guesses on why hooking up the vacuum line would have brought back all the issues?
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Old 02-18-2014, 08:28 AM
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Quote:
Originally Posted by 16Volt View Post
Does anyone have any guesses on why hooking up the vacuum line would have brought back all the issues?
My guess is that the car's mixture was tuned with an air leak in that line. Now that you've rectified the leak, the tune would no longer be appropriate. Just a guess. Your description doesn't sound like ignition related...it's not missing or backfiring or coughing, but sounds like it's hunting for the right AFR. Hard to diagnose from the written word.
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Old 02-18-2014, 09:09 AM
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Could your 'cruise' speed be right at the sweet-spot where the boost comes on?

My 70MPH revs are about 2900RPM, and the boost starts at 3000RPM, so on the hwy, I avoid that point setting the cruise control.

I don't know if disconnecting the O2 sensor is a good idea...won't the engine run rich and foul out your plugs/carbonize the tops of pistons, etc much quicker?
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Old 02-18-2014, 09:25 AM
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Cruise rpm should have nothing to do with boost until the air resistance (speed) requires an engine output beyond normally aspirated. You should be in vacuum when at cruise.

Double check the dual pot vac lines. Vac retard at idle does nothing harmful when unplugged, vac advance becomes purely mechanical when unplugged.
Surging indicates an air leak or mixture issue most typically. An ignition issue may clear up at higher RPM.
Verify the timing is set correctly and has not been set to compensate for this issue.
Old 02-18-2014, 09:42 AM
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I'm just parking this throttle body photo here for when you call 16Volt.

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Old 02-18-2014, 11:24 AM
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Awesome, thanks for the gut check people. It seems like the best course of action is to just keep going down the path with playing with the mixture, checking timing.

I am going to replace all the lines I can feasibly get to and go from there.

Getting my wideband hooked up I guess has moved into the top slot priority wise. Guess I will update my noob thread on that.

Can I submit my application for "least knowledgeable 930 owner" yet?
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Old 02-18-2014, 06:58 PM
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No... but you can submit it for "I told you this the week you bought your car" mebbe instead

the hole in the vac line probably confirms what Mark summized... that issue has been tuned 'around' and now you've fixed it so time to retune... timing / mixture

fret not, you'll get it... what's funny is how pretty much all of us were in exact same boat upon getting our cars
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Old 02-18-2014, 07:11 PM
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No... but you can submit it for "I told you this the week you bought your car" mebbe instead
Paul, you arse.

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Old 02-18-2014, 07:26 PM
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Can I submit my application for "least knowledgeable 930 owner" yet?
You might not know much now, but if you want to learn about the car's various systems, which it appears you do (unlike a certain someone I've recently had dealings with - not going to name any names - ROBBY ), and you get tired of making time and arrangements to take your car to a shop, you'll find yourself very knowledgeable before you know it.

And yes indeed - Paul is an arse of the highest order, and as such, is deserving of at least one of these per day -
Old 02-18-2014, 08:09 PM
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One of the things I like about this board is the G rating it typically maintains. Other boards are full of old idiots that act like 15 year olds. Please don't drag this board down to the ricer level.
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Old 02-19-2014, 04:07 AM
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Phew. Good thing I'm a young idiot acting like a 16yr old then... 8-)

I think the mebbe the sun's gotten to Ronnie's feeble noggin down thar? That or he's off / should find his medz? Recent spate of cheeky avatars (pun soooo intended) has been er uh um... revealing (I'm on a roll).

Two huge avatar thumb's up tho for the animated bird-flip'age! That one's a winnar!

(I think some/many may misconstrue Ronaldo & my ribbings of e/o, these are fueled by PM's that typically end w/ an affectionate "btw, FU" - but that's for me and we're just having fun, the assload of them elsewhere is otherwise befuddling)...

Shannon, def an arse - and def was just bustin bauls...
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Old 02-19-2014, 05:27 AM
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No worries Paul.

So on CIS-E in the VW world you can plug in a multi-meter inline with the plug on the fuel distributor and then set the mix using the mA value read on the meter. Since I have not seen it mentioned here is that not "a thing" on the Porsche setup?
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Old 02-19-2014, 05:57 AM
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i had the same miss/bump you have, except mine sounded like a fouled plug tryiing to fire.
2500 and at a steady speed. drove me NUTS. most people could not feel it but i sure could.

finally ran it down to an air leak on #3 and maybe #2. removed and resealed intake and that problem is gone.
mark had a good point about the mixture is off due to the air leak. check all your intake bolts and spray some carb cleaner around in there to check for leaks. some water in a spray bottle works too. then have the misture set again.

you can swap the dist retard hose to the advance port at idle to make sure the timing advances. if you can regulate an air compressor down to around 10psi, you can use that to check the boost retard. then just make sure the timing is at 26 degrees and leave it alone.

did JWcheck your control pressures? make sure they are correct. im sure he did.

my brothers O2 has been disconnected for years. never heard him say anything about surging or a miss. i think his idle mixture is at 13.5 if that helps.
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Old 02-19-2014, 06:19 AM
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