Vjgtrybno1

After last year's turbo swap into the S2 ('86 turbo motor), I hoped my engine issues would be minimal and they were - until a few months ago. The motor started idling rough and although there is no issue at WOT, maintenace or mid throttle produces a jerky, stumbling sensation - not a full buck, but definitely noticeable. I'm also getting much lower gas mileage. Here's what we have done so far:
1. Test and replace the oxygen sensor
2. Replace temperature sensor
3. Test and swap a new TPS
4. Swap an AFR and test
5. Swap a DME and test
6. Reset the idle using Clark's garage procedure
7. Replace reference and speed sensors
8. Replace the fuel rail with Lindsey Racing version (pressure is 3 bar and holds after engine shuts down)
9. Open the DME and inspect the circuit board - car has stock chips

Some of the above had minor positive impact, but none cured the issue.

We are doing a compression test this weekend and also swapping a KLR. Suggestions?

We are running at about 13 lbs. of boost and the boost guage shows some flutter at times.

Vjgtrybno1

Bump and update-
Compression was 125 across all 4 cylinders. Looking at the spark plugs, cylinder 1 and 2 were rich, while 3 and 4 were normal to lean.

Animus

Check the distributor rotor? Sometimes the screw comes loose if not tightened properly. I've also heard about people messing with the idle screw which throws off diagnosis..hopefully someone else can expand on this and help.

Vjgtrybno1

We're going to look at the wiring harness - to the injectors specifically. One pin controls 1-2 and another 3-4. Since we have different results on plugs 1-2 than 3-4, it makes sense to start there.

jlturpin

Injectors fire 1/3 and 2/4. No chance you re-used the flywheel fromt he S2? The flywheel and ring gear are much different between the S/S2 than in the 951. Since it runs well WOT, it could be a long list of things since it is running in closed loop while WOT and ignores some inputs that are figured in during partial throttle.

Vjgtrybno1

No, we did not reuse the flywheel from the S2, it was a total swap, except for the oil cooler. I am going to swap in the S2 oil pan, due to the crank scraper.

When you say injectors fire 1-3 and 2-4, are you referring to the order or where the signal comes from. In the thread below, it looks like DME pin 14 controls injectors 3 & 4 and 15 controls 1 & 2. Since the plugs for 1 and 2 are both looking similar and different than the plugs in 3 and 4, I was thinking we were onto something.

https://rennlist.com/forums/924-931-...re-colors.html

Vjgtrybno1

Ok - it was the KLR - swapped one from the esteemed Doc Boost and she runs like a top. But who knows why?

Keithr726

Maybe a chip problem or possibly the KLR was exposed to moisture? Any battery tray leaks?

Vjgtrybno1

No moisture issues... stock chips. I'd like to find out the exact cause in case it rears its ugly head again. We'll keep looking.

xsboost90

well when he and i looked at the wiring diagram this afternoon, looks like the injector wiring goes through the KLR- i would say something on the chipboard was fried.