1989 GMC Sierra won't idle

camarofanmn · September 29, 2006

OK. Got a weird one here:

1989 GMC Sierra, 5.7L TBI 4WD

129,000 miles, was running well until about 2 weeks ago.

The problems began with a slight stumble at cruise or idle (constant) rpms.

The computer threw a code 32, EGR system malfunction.

Truck ran fine for about 2 weeks after I saw the code, until about 2 weeks ago, when it wouldn't even idle. I can rev the truck fine in park, seems pretty responsive, actually.

After some research here, as well as other forums, I began thinking I had a vacuum or air leak somewhere. I have a Snap-On MT2500 scanner, as well as a 1992 Chevy with the same 5.7 TBI, runs great, so decided to compare some data. Everything between the two trucks checked the same, from MAP/IAC/temp/etc, except the '89 seemed to read lean constantly (at idle, also open loop).

If I hold the throttle partially open with my hand, the truck will hold a more consistant RPM, but still stumble slightly. I decided to try changing some of the obvious sensors betwen trucks around, and ended up swapping, one at a time:

MAP sensor

Ignition module in distributor

coil

New plugs/wires/cap/rotor

entire throttle body assemblies, including FPR/TPS/IAC

PCV valve

ECMs

checked out vacuum lines, all appear ok, did carb cleaner spray test around vacuum lines and intake.

replaced fuel filter

After all of this, I thought maybe I could have a leaking intake gasket, so I pulled the intake, had it resurfaced, and reinstalled it. All looks fine on that part.

The truck STILL has the same issues as when I started- will NOT hold a steady idle (i.e. surges so bad that it sometimes dies), until I can get it warm by holding throttle partially open. Once the truck enters closed loop mode, 02 sensor takes over and everything smooths out. Will idle fine, not sure about drive, since I don't trust it for a test drive yet.

Next step, I guess, will be to try to check idle fuel pressure and/or exhaust backpressure. I have NOT done a compression test yet, but doubt there are issues there, oil and coolant looked brand new when I changed the intake gaskets. I have not yet checked timing, but I cannot believe the timing would magically go AWOL until it hits closed loop, and then be fine again. Unless I am missing something on how the ECM operates in open loop.

If anyone can give me some advice on where to look next, it would be greatly appreciated!

Thanks in advance,

Seth

All parts removed from the '89 were tested on my '92, and all work fine.

GM-Tech · September 29, 2006

What are the integrator (long term) and block learn (short term) fuel trim values when it is running okay. How about IAC counts and MAP.

camarofanmn · September 30, 2006

OK- started the truck again and took some readings from the ECU:

In Closed Loop at idle of 1000 RPM:

IAC: 29

MAP Hg: 7.8

INT: 140

BLM: 172

O2 crosscounts: varying between 2-13

Coolant temp: 206

System voltage: 14.0

ESC: No

Knock: No

A/F Learn: Yes

AIR Div Sol: Div

Air Switch Sol: Conv

TCC apply: No

It looks to me like the computer is still seeing a lean condition, with the BLM's that high. Idle is higher than when I last saw it too, it was 825-850 earlier. I have a hard time believing that the issue is due to any kind of internal problems with the engine, as once I get into closed loop the truck seems to smooth out considerably, with the exception of the high idle.

Any ideas?

One other question I have is: could there be an issue for some reason with ignition controls once closed loop is reached? Seems like there wouldn't be, but what do I know...

Thank you again!

Seth

GM-Tech · September 30, 2006

MAP looks out of whack, but I think you said you swapped that. How 'bout the hose feeding it?

We had an '89 or '90 last year that just whipped our butt. MAP readings were screwy and didn't match voltage readings taken right at the ECM. After trying a MAP, new connections, etc., we replaced the ECM with no change. To make a long story short, it ended up being the PROM. I'd say that in all the years I've worked on these things, I never saw a bad PROM before.

camarofanmn · September 30, 2006

Yes, I swapped MAPs between the the two trucks and it didn't seem to do much for it. Also, when I swapped ECM's, I did the "no-no" and left my PROMs in the original ECM, so that would almost make me think the ECM & PROM are both fine. I will try looking into the MAP thing again. It sure seems weird, though, that the MAP would cause that many issues during open loop and not so much during closed.

One other thing I noticed is that there is a noticeable "whistle,' or sound of rushing air coming from the throttle body, where the IAC sits. In my '92, it doesn't make any distinguishable noise, but the '89 is LOUD...

Thank you again,

Seth

BTW: what are the O2 crosscounts I'm seeing doing and what effect do they have?

GM-Tech · September 30, 2006

One other thing I noticed is that there is a noticeable "whistle,' or sound of rushing air coming from the throttle body, where the IAC sits. In my '92, it doesn't make any distinguishable noise, but the '89 is LOUD...

BTW: what are the O2 crosscounts I'm seeing doing and what effect do they have?

<{POST_SNAPBACK}>

Almost sound like a TBI base gasket, which these are famous for but you say you swapped the TBI and you couldn't have missed that... right? Some of these just sound that way, so it may be insignificant.

Crosscounts are how many times the O2 swings past the bias voltage level in a given period of time. I've never found this information to be useful.

camarofanmn · October 3, 2006

OK... Once again, back with more details:

If I can keep the truck running for several minutes (throttling the tb manually),

it will eventually stay running on its own, albeit very roughly. Once the truck goes into closed loop, INT and BLMs skyrocket to 140/172, repectively. IAC closes from ~140 to near 0, and truck evens out. MAP/temp/02's seem to be ok. I went back around everything with a can of carb cleaner, and no results whatsoever.

Could this possibly be a low fuel pressure issue? If so, how in the hell do I check it? I can/will make an inline adapter, but what threads does this thing use? AN?

I understand it should make 9-13 psi, and would like to verify it.

I also wanted to check base timing, just in case that would have an effect at idle, but as soon as I pull the connector the truck won't run. I honestly believe it's not a common sensor issue, as these have again all been swapped from a running vehicle.

I hooked up a manual vacuum gauge (T'd off of the MAP sensor), and get consistent readings of ~20 in/Hg at closed loop idle when running. Still idling high when it smooths out, though.

I am at a real loss here, and just want to get this monster back on the road!

What are your thoughts, Oh GM Masters? ;)

Thanks,

Seth

GM-Tech · October 3, 2006

Once the truck goes into closed loop, INT and BLMs skyrocket to 140/172, repectively. IAC closes from ~140 to near 0, and truck evens out.

I know you've done everything in this regard, but dang, these sure sound like a vacuum leak somewhere.

Could this possibly be a low fuel pressure issue? If so, how in the hell do I check it? I can/will make an inline adapter, but what threads does this thing use? AN? I understand it should make 9-13 psi, and would like to verify it.

I thought pressure had been checked. That's definitely where I'd want to go before doing anything else. You have the specs right, but I find when they get near the 9 psi range, it's no good.

Yes, you need an adapter. It normally goes in at the fuel filter connections. One of the guys here made his own adapter out of an old fuel filter. He just screwed a connection into the body of the filter, then replace the on-car filter with this one with a gauge screwed into it. You want to be careful of any welding or soldering on this thing because it can become a mini-bomb!