Reposting to this board in hopes of getting a reply. I really feel like someone that is knowledgeable would be able to spot my problem in the data below. The surge is like clockwork (you can tell from the SCAN session). It's almost like something is turning on in cycles and killing the RPM's?

Surging idle problem - help with the data I've found
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

 

IAC counts changing?...if so, why?

 

Should they not be? I'm not sure out of those inputs which ones are rising/falling that shouldn't - is the IAC the one?

 

im having the same problem, code 13 now on my 1987 5.0 tpi. My book says check timing, mine is at 2* advanced. I'm curious what yours is at?

i've replaced injectors, fpr, tps, maf, egr, coolant temp sensor, intake gaskets, vacuum lines, fuel filter, & full tune-up. it all made the idle sound cleaner but it still surges sooo bad

 

Not sure how to check the timing, I'll look that one up. My surges are like clockwork: slowly dying rpms, surge to the high, then slowly down again.

Checked my IAC and it's doing the right thing. The RPM's are dying while it remains in the same position, then closes when the surge happens. One think I noticed is that the fuel pressure remains steady also while the rpm's die then jumps up a few pounds for a short burst over and over again.

 

I think it is doing the right thing - iac closes (counts up) when the surge happens. Shouldn't it? Checkout the surging data log snapshot above.

 

You need to locate the EST bypass connector...on my '89 it's near the p[*** side strut tower base...disconnect it...and then with a light, set base timing at 6?...I think...for stock setting.

Shut car off, reconnect the est bypass...and you should be good to go.

You'll have a code 42? I think, set in the ECM..but not worries

Edit:

Hmm..wo't let me put in passenger up top..