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

 

surging idle is a classic sign of a vacuum leak. I know you changed the egr valve but make sure its not staying open. Do a search, it's a common issue.

 

Should look at your IAC steps to make sure its functioning also.

I would also check for vacuum leaks. Lots of ways to check, smoke, brake/carb cleaner, propane & hose method, hose to ear method... sounds like your problem.

You could always get an IAC motor from the parts store, hook it up and see if it makes your problem go away. If not, take it back.

 

No vacuum leaks, IAC is doing the right thing - you can tell from the data log sessions above. When the RPM's drop the IAC is steady, and it closes when the RPM's surge from lowest to high.

What about my fuel pressure observation? Fuel pressure is also steady while the RPM's are decreasing then jumps up a few lbs when the idle goes to high?

It seems to do this like clockwork. Slowly idle down, chug, surge, slowly down again.

 

The scans were at idle?

Strange...the TPS voltage is on an average lower than spec, but also changes?...TPS voltage should sit rock solid at .58 or .6...can't recall, but definitely should not be fluctuating at idle.

 

TPS is set to 0.54Volts +/- 0.075
WOT should register more than 4Volts with a steady increase while opening the throttle.
8mike9, the TPS in pic 3 is the bottom green line, doesn't seem to be fluctuating but it may be set a little high. I would check it to meet the above criteria first.

I replaced a stock FPR diaphragm on my car a long time ago. As I recall the part was like 6 bucks. Might be worth your while to get one and replace yours.
Sucky part is the plenum has to come off.

 

Must be the screen read out and the colors I am interpreting.

Why can't data logs be graphed like a dyno test?

 

Sorry, what is the FPR? I just took the plenum off to replace the EGR so am very familiar with the process . Oh, Fuel Pressure Regulator?

Not an expert obviously, but should the fuel pressure always remain constant or is it expected to surge like I'm seeing when the ECM is trying to keep the engine from dying (which I think it is when the RPM's dip low)?

 

I did a propane test and didn't get any RPM increase, but not sure I checked in all the places I should. Any advice on what locations I should point the propane hose to around the engine?

 

TB to plenum, plenum to runners, runners to intake manifold, injector bosses, intake manifold to heads, EGR valve, vacuum lines.
That is just about every place you could possibly have a leak.

Don't forget to spray propane (or what ever you use) into the front of the intake with the car running. You need to get an idea of how it affects the engine so you know what to listen for.

to check fuel pressure: