Can someone that is familiar with all of these parameters take a look at my log and see what you think? It is like clockwork - engine starts at 1200 RPM, slowly dies to 700 RPM and races back up to 1200 quickly. I've included the post I put on the other boards, but am really looking for feedback on the attached logs.....

THE PROBLEM

Just learning my way around the engine and trying to diagnose our 1986 5.0 TPI F Automatic IROC w/ AC.

- the idle goes from 1299 down to 700 every 30 seconds
- RPM's fall slowly down to 700 then immediately jump back up to 1300

I'm attaching the SCAN monitors from RTTunerPro with two timescales for the same session.

WHAT's BEEN DONE SO FAR

FYI, it was previously throwing 21 TPS and 34 EGR codes intermittently. The EGR valve was bad and TPS needed to be adjusted. After doing that (and cleaning the TB and Plenum it is running much better), but still having the surge/die idle problem.

FUEL PRESSURE TEST

I did a fuel pressure test w/ the following results:
- key on: 44 PSI settled at 42 and held
- running: 36 when idling**

** With the gage hooked up, the following happens:
- rpms dropping, fuel pressure fairly steady, then jumps to 40 for a sec or two when the idle goes from 700 to 1300

SCANS BELOW

- hid the o2 sensor stuff, it's cycling fine

- FIRST PIC
- shows the wide view of the scan session, you can see the sawtooth RPM's

SECOND PIC
- after start you can see the surging RPM's
- Don't understand exactly what the BASE INJECTOR PULSE WIDTH ms(green) and BASE INJECTOR PULSE WIDTH CORRECTION ms (purple) are and how they relate (could someone explain), but it seems they cross each other right when the surge happens to get the RPM's back up

THIRD PIC
- this is after giving it some throttle for a bit and running at 2000 RPM's
- idles great after that
- you can see the two values above are all steady (although the BASE INJ PW is a bit jittery)
- one odd thing is the TPS voltage is now at .61 when not touching the throttle

Does this happen at all temperatures, or only after it warms up? The log shows the coolant temp in the 150-160F range.

After you revved it, I agree it looks odd that the TPS didn't drop back to the initial value of 0.55. Perhaps the TPS is a bit inconsistent, but I don't think it should cause this type of idling problem as long as it isn't moving while the problem is occurring.

I've heard of this type of hunting idle being caused by bad grounds, and sometimes by vacuum leaks.

The Base Injector PW should be the calculated pulse width that the ECM is commanding for your injectors to open. The Base Injector PW "Corr" is probably an adjustment to this figure, used in closed loop mode based on feedback from the O2 sensor. I could be wrong about that, it's just an educated guess. I'm not used to MAF systems, my car is MAP based and it doesn't have that parameter. I tried some searching but I didn't find a detailed explanation of the "Base Injector PW *Corr*" value.
I don't know if that Correction factor is already included in the "Base Injector PW" value, or if it's added later. From looking at the graph, I get the feeling they may be separate, and added later.
My guess of why this happens:
I think your Corr value is generally low - giving the engine less fuel than what's normally calculated. When the engine RPM gets low enough, your ECM decides it needs to increase the Corr (giving it more fuel). This momentarily allows the engine to gain RPMs, but then your ECM cuts the Corr value back down again.
As your RPMs increase, the calculated pulse width (base PW) would go down because there are more cycles per second occurring, so the length of each event can be shorter. It seems a bit backwards but I've seen the same pattern on my car when revving without a load.


I'm thinking your PW Corr value might be similar to the INT value on speed density. I notice that your Corr value is low when it's hunting, and higher when it's idling properly. On my car, my INT was very low when it was hunting, and higher (in normal range) when it was idling properly. So this is a similarity, but on the other hand, mine didn't clear up after revving, and it looks like yours did.

It's possible the O2 sensor is fooling your ECM by reporting falsely rich, but I don't think that's common.

Does your car have stock injectors? Do you believe them to be in good condition? Idle is sensitive to the lower fringes of injector behavior.
Your symptoms remind me of a stumbling/surging idle I had on a 2.8L Fiero. In my case, the problem only occurred after the engine had warmed up. Blipping the throttle would stabilize it for a few seconds, then it would start hunting again. Putting a load on the engine (even as little as turning on electrical accessories) would also stabilize it.
My logs showed that my ECM was gradually leaning the mixture (dropping the INT value), and when INT got sufficiently low the surging would start. Occasionally the ECM would panic and richen the mixture for a while, but about 30 seconds later my INT would get really low and it would start hunting again.
- that problem turned out to be a miscalibration between the ECM and the non-stock injectors I had in it. An ECM tuning adjustment fixed it.


This sounds normal to me. The fuel pressure regulator maintains a constant differential versus manifold pressure.

check for vac leaks. If the hoses are old may want to replace or inspect them thoroughly. A split is hard to see. Spray the manifold with carb cleaner or some use propane.