So long story short this 86.5 was purchased as a non-runner, PO had been messing with it all over.

Work that has been done under my care:

Timing Belt water pump, full intake re-fresh, cam pads and so on.

The current issue is this: Car runs perfectly, idles smooth and rev's well. But after about 5-7 minutes the throttle response total dies off, the car will still idle but starts to suck air and blow darker smoke if I rev it. I did disconnect the Maf then the throttle response came back but not fully, then it started to blow black smoke so I shut it down. After shut down it will not start due to lack of spark but injectors are firing and I can smell the fuel. Once cooled off it will start again and with the same cycle. I have swapped both the LH and EZK but I'm not able to test those computers in my 87. I have swapped out every relay, and swapped out the Maf with the one from my 87 but it still would not start. Oddly when I pulled the fuel pump relay it would then start with the fuel that is in the rails, but then dies due to lack of fuel, I popped the relay back in and it still would not start (no spark). I swapped that relay from my 87, still no change. I gave up and walked away, about 30 min later it fired right up, but then again after about 5 mins the same issue returned.

I have checked fuel pressure and I have 32lbs with engine off, and while running or revving. I did not have the guage on during the running issue, so I may need to repeat that test. CPS is new, TPMS new, Temp II new and so. All wires for these check out fine at the LH.

Maf looks to be out of spec. Hotwire resistance is only 2.5 ohms and should be at least 4 ohms. The voltage from MAF at LH was good at 1.7v. The Maf is a rebuilt unit, no idea when it was done.

My next plan is to pull the MAF from my 87 and try a cold start with it in the 86.5 to see if the same issue appears. The only item I have not checked out fully is the ignition amps, is it possible that once warm one or more of the amps are failing after a few minutes? I have done spark tests, but probably need to do another spark test when the issue arises to confirm.

Anyway, I'm focusing on the MAF and Ignition amps next. But would love to hear thoughts from the brain trust in case there is something I'm missing.

 

The basic MAF output is what the car uses to run when the engine is cold, so the fact that it runs great, when cold is a positive "sign" for the MAF.

Once the car warms up, it uses the O2 sensor, to alter the information from the MAF, to correct the mixture.
Simply disconnect the O2 sensor when the car starts running poorly and see if that makes the car run good.
If that doesn't change how the car runs, the water temperature sender alters the mixture...and can have problems. Check the connector for crossed wires under the boot and corrosion at the connection to the sender.
If you still have an issue, check the continuity from the sender directly to ground. Each terminal needs to have the same resistance. Check the resistance and compare it to the values in the workshop manual. Make sure you check this, when the car is warm and is running badly....one side of the sender open or shorted to ground can cause problems.

And remember to always buy your parts from the company that offers free technical assistance...Precision Motorwerks.

 

^ Yes, I already disconnect the O2 sensor and it did not have any effect on the running condition. I will double-check the Temp II readings when warm, the harness is in decent shape, no shorts that I can find.

 

Could the Ignition modules over heating cause this issue? If the heat sink grease on the back side has hardened and lost its ability to reject heat, could this cause an intermittent issue like this one?
I don't know that it will but just throwing it out there for the brain trust.

 

I was thinking the same. When I had it running on Monday it clearly had no spark when I tried to restart it. It's an easy swap from my 87 so if my Temp II readings are correct when warm I will look at the ignition amps.

Failing all of that I will send the EZK/LH/MAF to John speak for testing.

 

GB for the win. Temp II sensor is fubar.

New sensor in and running like a champ.

 

For an S4 at least, that fuel pressure is too low.

 

S3's run lower pressure and 24lb injectors. LH 2.2. S4's with LH 2.3 run higher pressure and 19lb injectors. At first I thought it was low as well as my brain was thinking S4 numbers.

 

Glad you found the issue,

FWIW also the TPS short harness has the crumbly connectors,
if yours has not been replaced then you should swap in a new part,
Roger has them

 

Ya, it has a new TPS harness. Going to take her out for test drive later today.

 

Thanks for the heads up.

And congrats on fixing your car!