Rough idle, Timing randomly adjusts

[ChevPasmore]

Hello,

I have a 1990 GMC with a swapped in 95 5.7.
Using factory harness from the 1990 and accessories.
I have replaced distributor, cap, rotor, ignition coil plus brand new timing chain as of 1 week ago.
The timing still randomly drops and I have to reset it every time. Could this be caused from a faulty knock sensor or IAC?
When she runs, she runs good. But if I drive it for a bit, then shut it off and go to it say 3 hours later, it has a rough idle, and wants to die and I have to advance the distributor to keep it running then re-time it.
Yes I know to get it up to temp, then to disconnect the timing advance and set it then plug it back in after I shut the vehicle off.
No known vac leaks that I can find, throttle body is firing nicely. RPM's do not drop at all when engine warms up which leads me to believe the IAC is faulty.

Any insight would be great.

Thanks,
Chev.

 

[Schurkey]

If you have to turn the distributor to re-set the timing...


...the distributor is not getting tightened properly.

Is the hold-down bolt too long? Hold-down clamp bent? Heads, block decks, or intake manifold machined excessively leading to lack of clearance between distributor and oil pump driveshaft?

Figure out why the distributor is moving, and correct it. THEN worry about the IAC. When it comes to sensors and computer outputs, the scan tool is going to be your best friend.

 

[ChevPasmore]

If you have to turn the distributor to re-set the timing...


...the distributor is not getting tightened properly.

Is the hold-down bolt too long? Hold-down clamp bent? Heads, block decks, or intake manifold machined excessively leading to lack of clearance between distributor and oil pump driveshaft?

Figure out why the distributor is moving, and correct it. THEN worry about the IAC. When it comes to sensors and computer outputs, the scan tool is going to be your best friend.

It’s clamped down properly. I have to loosen it and turn it a bit to bring the idle up to stop it from running like crap. Then after she’s warm, I have to unplug the connector, reset the timing to 0 degrees tdc. Then she runs mint. But again after that, if she sits for a few hours it does it all over again. It is a stock engine. Nothing machined excessively. The original chain was loose, so. I replaced it with everything else for good measure. Replaced coolant temp sensor and 02 sensor as well. Distributor is not 180 out or skipped a tooth. So at this point I am stumped on what’s messing with the timing or what’s causing the rough idle and for it to not stay at the current timing position.

 

[Schurkey]

Connect a scan tool, verify IAC and knock-sensor data.

While you're in there, verify EVERY sensor and computer output.

Make sure the wire harness from distributor to ECM is undamaged, including the wire that's disconnected to set base timing.

 

[ChevPasmore]

Connect a scan tool, verify IAC and knock-sensor data.

While you're in there, verify EVERY sensor and computer output.

Make sure the wire harness from distributor to ECM is undamaged, including the wire that's disconnected to set base timing.

Unfortunately my OBD port has no power...Not sure where the ground or power cable run to. I suck at electrical work hahha.
All wiring is undamaged as far as I can tell, no breaks or tears. I will pull the loom further back to inspect more though.
I appreciate the help.

 

[tayto]

what engine was in the truck originally? i assume you haven't installed the correct calibration in the ECU or the correct injectors?

 

[ChevPasmore]

what engine was in the truck originally? i assume you haven't installed the correct calibration in the ECU or the correct injectors?

It was originally a 5.0... I did not swap the engine. I bought it with the 5.7. Ran good then after a month it crapped out.

 

[tayto]

so have you verified that it has the correct calibration and injectors? i am not saying this is your problem 100% but a lot of small problems compound to a bigger one.

EDIT: not trying to start a pissing match but your/most people's definition of "runs good" is probably vastly different to someone who does this for a living or knows what they are doing. usually "runs good" says to me it just "runs" not necessarily means how it's suppose to run....

 

[ChevPasmore]

so have you verified that it has the correct calibration and injectors? i am not saying this is your problem 100% but a lot of small problems compound to a bigger one.

EDIT: not trying to start a pissing match but your/most people's definition of "runs good" is probably vastly different to someone who does this for a living or knows what they are doing. usually "runs good" says to me it just "runs" not necessarily means how it's suppose to run....

No pissing match here. By runs good, I mean it ran good with zero issues, for the first bit I owned it. No hesitation, no checkin engine light etc. then out of the blew one day driving home it died twice on me. Fuel pressure is good, injectors are shooting fuel like they should. Checked the timing chain slack and found out it was loose. So replaced all the components stated previously. But now she will turn over but severe rough idle. Even after I set the timing, it would idle fine but not idle down after warm. Then as I said I let it sit for a few hours then it’ll have the rough idle again. So something is messing up. It could be the ecm and prom not meant for the engine. But only way for me to pull up codes is by finding out how to get power to the obd port. No dead fuses. Not sure how to install the correct calibration?

 

[tayto]

what does "fuel pressure is good" mean? what psi is it? Just because injectors are firing and the spray pattern looks good, does not mean they are good or correct. Pull out the ECU and open the small cover, the memcal will have a 4 digit code on it. Post that here and we can see what calibration is in it.

I got my first TBI vehicle was 20, 1989 Caprice ex-cop car w/ 5.7L. For starters it "ran good", but as I discovered after a year or so of owning it, it had been fixed improperly. For starters someone put a 4.3L TBI on it with 4.3L injectors. Putting the correct injectors in it made a huge difference in how it ran. Then I changed the fuel pump (this was before I got a fuel pressure gauge) and upgraded to a TPI pump, again huge difference. fast forward about 4 years I moved to an area where annual emissions testing was required and car failed miserably. At this point I finally invested in a fuel pressure gauge and a datalogger. Here's what I found:
- fuel pressure was 8-9 psi (technically in spec)
- the BLMs were maxed out, so it was running lean

I made the fuel pressure regulator adjustable and got the pressure up to 13 psi. Made a big improvement on how it ran but still had maxed out BLMs. I checked timing, minium air adjustment, valve adjustment, resealed intake, etc and still ran lean. As a hail marry i went to the wrecker and got some injectors out of a 5.7L and threw them in. BLMs immediately went to 128 +/- 2. Perfect. Passed emissions testing with flying colours. With all the other repairs combined it now made better power, smoother idle/exhaust tone, drove better, etc than it EVER had. This is why I always verify these things and how a fuel pressure gauge and scanner/datalogger are required for proper EFI troubleshooting.


BTW, the caprice never had a CEL either until around the 5 year mark and it would occasionally get a lean o2 code.