MFI troubleshooting - high idle, pops and bucks at part throttle

[Arne2]

Car - 1972 911T 2.4 MFI, engine all stock

Background - Years ago (at least 3 owners ago) MFI was swapped to carbs, but parts boxed and retained. Previous owner had engine rebuilt (all stock spec) and MFI put back into service. Project stalled immediately thereafter. I bought it as a running car, but with only 2-3 hours on the engine, and no road testing (due to bad brakes).

Now that the car is road worthy again, I'm looking at some of the basics in order to correct running problems. Main problems are high idle speed (~1300) and popping and bucking at part throttle. Smooths out and pulls nicely at wider throttle opening. These problems only surface when the engine is fully warm. When cold or during warmup neither symptom is evident.

Keeping the old Check-Measure-Adjust doc in mind, the ignition and engine bits are good. New air filter, plugs, points, plug wires, good compression, fresh valve adjustment. Timing appears good, and the vacuum retard is working as expected.

So I need to look at getting the idle speed down, and perhaps richen(?) the part throttle on the pump.

So first I look at the idle. And it is my understanding that the idle is adjusted with the 6 air screws on the stacks, and then the synch is re-checked.

But there is my first failing. No amount of changing on the air screws appears to have any effect on the idle speed. The screws were all over the place when I started, after trying to drop the idle speed for a bit with no result, I stopped the engine, screwed all of them in to the stop, then backed each out 2 full turns to have a starting baseline.

Still idles at 1200-1300 when warm. All of them check fairly close on my synch tool, but still no idle change.

Any thoughts?

[Jonny042]

Linkages disconnected when adjusting screws?

Edit - also might be worth checking if the idle air passages are free - I recently rebuilt a set of throttle bodies where they were all completely plugged.

It might be worth starting from scratch, taking the throttle bodies off, checking them out off the car, resetting the stop screws, etc. since you don't know what sort of understanding the previous mechanic may have had.

[stownsen914]

Re: the high idle, if closing the air screws entirely doesn't improve the situation, then air is getting through some other way (and unclogging those screw passages won't fix it). I'd check to make sure the throttle plates are closing fully. As mentioned above, you can disconnect the linkages to test this, to make sure nothing in the linkage, throttle cable, etc. is hanging it up, and adjust those as needed. If disconnecting the linkage doesn't fix it, there are threaded stops on each butterfly mechanism that can be adjusted to close the throttle plates more. (If you mess with them, keep track of how many turns you do on each one so you can undo your changes if needed.)

How sure are you that you don't have air leaks where the throttle bodies mate to the heads? Any leaks there will let unmetered air in and mess up your A/F mixture and idle settings.

Re: the popping and bucking ... you have the warm up circuit hose plumbed correctly from the heat exchangers to the pump? This is very important to make sure the A/F mixture is correct. If you don't have air leaks and the warmup circuit is correct, this would point to the mixture settings on the MFI pump (or possibly its condition), but it may be hard to give solid advice until you confirm no air leaks and that and linkage is set correctly.

[tobluforu]

Since this car was a carb car, does it even have the correct HE for a MFI engine?

[BK911]

Quick way to test for lean or rich condition at idle is to disconnect the rod from the throttle body to the pump then open the throttle body slowly.
If idle goes down, too lean.
If idle goes up, too rich.
If idle goes up slightly then goes back down, perfect!

[r lane]

Does sound like the warm up is not working correctly, which is the hose going to the thermostat from the left side sheet metal. Has to have the correct heat exchanger for this. I have built them for headers. As one responder stated, make sure the idle ports are clear, and then start with the throttle plate adjustment. Try to do that warm, bottom out the butterfly and then open a qtr or half turn. Make fine adjustments with the air screws.

[Arne2]

Actually, I'm pretty sure the warm up thermostat is working. It is correctly plumbed to the HE, and while cold (thermostat pulling to rich for cold start) it runs reasonably well. As the engine warms and gets up to temp, it gets worse as the thermostat drops to normal (full lean). So I'm not too concerned about that part.

So far I have verified the main pump control rod is correct at 114mm, and the pump is timed correctly to the cam. Going to warm it up later today, then reset the throttle plates.

[john walker's workshop]

You need a tailpipe sniffer to see where you're at. They like to be on the rich side both at idle and midrange/3000.

[Jonny042]

Originally Posted by john walker's workshop

You need a tailpipe sniffer to see where you're at. They like to be on the rich side both at idle and midrange/3000.

Wideband O2 sensors are so cheap and easy these days......Ebay price for an Innovate MTX-L is less than $160.00. Thats for the gauge, controller, and sensor.

[tobluforu]

I saw that you are still having problems. Years ago my 72 did somewhat the same thing, but worse. It ran great when I started it, then ran super lean after it warmed up and was hardly drivable. I spent weeks trying to fix it myself and finally brought it down to my mechanic. 1 hour later he called, fixed! The rubber fuel line under the car running to tank was constricting because of how old it was. Check all of your fuel lines, that is if they are not already new.

[Arne2]

Thanks for the reply. I am actually focused on fuel delivery now, and will be doublele-checking all that. The fuel lines that I have inspected so far are fresh and new, but I haven't yet looked at them all.

[Arne2]

Just to touch back on this topic, the fuel lines are—as I thought—all new. But I did find a problem, the check valve at the filter console for the return line is bad, and is stuck part way open. This would cause insufficient fuel pressure for the MFI pump. It will need to be replaced before I can go any further.

[cmcfaul]

maybe a stupid question but is the hand throttle all the way down?

make sure heater hose is connected to MFI and HE

lower idle by closing all the throttle bodies and resynchronizing them.

Then adjust fuel mixture at idle.

Then adjust fuel mixture at WOT and see what happens at partial throttle. This can be done easily by adjusting the linkage pushing on the MFI. Or through the adjustment screw.

Make sure timing is set correctly and advancing to spec.

AFR meter is essential

All else fails. Send the unit out to be rebuilt and calibrated while there are still people that know how to do it.

Chris
73 E

[Arne2]

Originally Posted by cmcfaul

lower idle by closing all the throttle bodies and resynchronizing them.

Then adjust fuel mixture at idle.

Then adjust fuel mixture at WOT and see what happens at partial throttle. This can be done easily by adjusting the linkage pushing on the MFI. Or through the adjustment screw.

Make sure timing is set correctly and advancing to spec.

Wrong order, Chris. All ignition and other non-fuel things need to be correct before doing anything to the injection, per Porsche's Check-Measure-Adjust procedure. And idle mixture should never be adjusted until AFTER the part-load (main rack) adjustment is done and correct.

[Jonny042]

Originally Posted by Arne2

Just to touch back on this topic, the fuel lines are—as I thought—all new. But I did find a problem, the check valve at the filter console for the return line is bad, and is stuck part way open. This would cause insufficient fuel pressure for the MFI pump. It will need to be replaced before I can go any further.

Something you might find interesting, post # 453 and onwards on page 23 of my build thread:

Project Heavy Metal - an all steel classic, remastered

It goes into some detail about the intricacies of the MFI fuel delivery system. Some mention of the pressure regulating fitting (which I assume is the what you are referring to as the check valve) as well.

If your car "cleans up" and runs well at higher throttle openings and rpms I doubt you have a fuel delivery issue. The MFI pump is pretty forgiving with that actually.

Have you checked your injectors? There is a huge difference between the way a new one and a grungy old one behaves, and the difference would be most pronounced at idle. Not a cheap fix but one less thing to wonder about. I wouldn't waste time trying to clean the old ones.

[Jonny042]

Originally Posted by cmcfaul

All else fails. Send the unit out to be rebuilt and calibrated while there are still people that know how to do it.

Chris
73 E

Yikes! Is the situation that dire? I know the main suspect, Gus, is no spring chicken and must be close to retiring..... there seem to be a few others scattered around but my impression is they are pretty busy.

[stownsen914]

Originally Posted by Arne2

Just to touch back on this topic, the fuel lines are—as I thought—all new. But I did find a problem, the check valve at the filter console for the return line is bad, and is stuck part way open. This would cause insufficient fuel pressure for the MFI pump. It will need to be replaced before I can go any further.

Good find. These check valves do go bad, and are critical for an MFI car to run correctly. The MFI pump needs 15 psi. If that check valve is bad, you might be getting half that if you’re lucky. I had one go bad on my track car years ago and was getting like 10 psi, and car ran like crap. Fixed the fuel pressure (I used an external unit since originality wasn’t important for my application), and all better. Definitely fix that before you bother doing any other troubleshooting.

Scott

[cmcfaul]

Originally Posted by Arne2

Wrong order, Chris. All ignition and other non-fuel things need to be correct before doing anything to the injection, per Porsche's Check-Measure-Adjust procedure. And idle mixture should never be adjusted until AFTER the part-load (main rack) adjustment is done and correct.

Fair enough. Was not listing in order

[Arne2]

Follow up - 6 pages of diagnosis, brainstorming and suggestions in a thread over on the Early S Registry. MFI troubleshooting - high idle, pops and bucks at part throttle

Spoiler Alert!! After much head scratching it was discovered that the 2 inlet lines at the fuel filter console were reversed. Car runs great now.

[Jonny042]

Glad you got it sorted!

So without reading 13 pages of posts, the short version of the story is you didn't check the fuel pressure at the MFI pump head?

I'm sorry I wasn't following more closely, that's a pretty basic step, and I'm surprised nobody suggested it!