95 TBI Rough idle first 45 seconds

[The Iron possum]

problem: 95 K1500 with 5.7 TBI 4l60e 197000 miles stock with stock replacement parts. Recently replaced ignition control module due to miss under load. Heat sink paste was like concrete so figured it fried. Now, when engine starts cold, it idles very rough up and down between 600 and 1200 RPM and almost dies. Very rich fuel smell with lots of smoke. After about 45 seconds, idle smooths out and runs great. If I start with timing advance bypass wire (black/tan) disconnected it starts and idles fine. I have done all of the Haynes checks for Reference voltage and voltage with key on and everything checks out in tolerance (map, iac,cts,tps). If I unplug the IAC, and start, idle goes to 1500, but is smooth. Stays there when I plug it back in, but goes back to rough on next start. IAC is <1 yr old and resistance check on pins is good. No vacuum leaks on EGR tubing or changes with starting fluid spray around hoses and gaskets. New plug wires and another Ignition Control Module with no change. No SES codes either. Runs fine after first 45 seconds. I don’t think that’s enough time to go closed loop. Has 3 wire O2 sensor. Any thoughts on what else to check. Ive, seen similar posts on this forum, but most end without giving a solution.

 

[Supercharged111]

What kind of ICM did you go with? Parts store ICMs are ******* garbage. That said, be sure to check your fuel pressure. Down below zero, my 88 was garbage for the first 15 or so seconds, but after that it was OK at an idle but if you tried to roll right away it was an effort in patience.

 

[The Iron possum]

ICM is Duralast. I’m going back into the distr. today and will change to AC Delco. An old thread I found talked about being careful with heat paste application on ICM and that you could lose the ground if the mounting screws were covered in paste. I applied paste liberally for sure both times and have it on screws. So I’ll clean that up just in case. Will have to order fitting to check fuel press. Fuel pump replaced last year with AC Delco, so it is fairly new.

Anyone know which inputs the ecm looks at on startup or does the fuel/spark run off a preset table until the ECM gets booted up? If it runs off preset, what tells it where in the table to run?

Thanks for the help.

 

[Supercharged111]

Not sure on OBDI, but it definitely uses VE as an initial part of the calculation. For OBDII it will also have a cranking timing value separate from the running timing table. Next it will have an ECT enrichment table and a start enrichment table, but the start enrichment will be tapered off. The OBDI stuff is a bit more rudimentary, but will be very similar.

 

[The Iron possum]

Can someone describe which inputs the PCM looks at when the engine is first started? I’m wanting to know what it looks at to determine injector pulse and timing advance right after starting until PCM boots up and starts looking at other inputs. Still troubleshooting a rough idle that smoothes out approximately 45 seconds after start. Almost like PCM is using wrong VE table on startup. Truck is stock 350 TBI with 4l60e, 197K miles.

 

[Lattimer]

I had the exact same issue on my 94 Blazer. Rough idea and almost stall for about a minute and then clear up. I bought the truck acting this way.

PO had put in a complete new parts store dizzy and new wires. Coil looked old. All of the sensors also looked new and I think the TBI is a rebuilt unit. Old dizzy, sensors, and TBI were in a box in the back.

I ripped the ignition out and put in a DUI Distributor and coil and new wires, with new AC Delco plugs. Problem cured. Pretty sure it was the junk distributor.

Not a cheap fix but now it runs really good. 201k on it.

 

[grampadirt]

Can someone describe which inputs the PCM looks at when the engine is first started? I’m wanting to know what it looks at to determine injector pulse and timing advance right after starting until PCM boots up and starts looking at other inputs. Still troubleshooting a rough idle that smoothes out approximately 45 seconds after start. Almost like PCM is using wrong VE table on startup. Truck is stock 350 TBI with 4l60e, 197K miles.

Don't know if this helps,from ALLDATA: In "Open Loop," the PCM ignores the oxygen sensor voltage and bases the amount of fuel to be delivered on TP sensor, engine coolant, and MAP sensor inputs only.

What condition is the coolant temperature sensor in? Very important.

 

[The Iron possum]

Coolant sensor ohmed out to match temperature chart when cold and hot. I pulled and cleaned ground also and then replaced it for good measure with no change. Also pulled and cleaned other manifold grounds.

Replaced EGR since it was original and symptoms matched a stuck open EGR valve. No change.
I don’t normally throw parts at problems without narrowing down the issue,but most parts have autozone warranty so it’s free to exchange them.
All volt/ohm values compared to Haynes manual
New plug wires (pulled 2 out of boots removing)
New ignition control module. Heat paste and grounding screws carefully installed.
Checked ICM wires for continuity to plug
Pickup coil ohmed in spec
New Idle Air Control valve
Checked knock sensor - ohmed out in spec
Checked EGR vacuum lines, no leaks, had parts, replaced 2 anyway
TPS voltage check from close to WOT in spec
MAP sensor voltage check (not running) in spec
Replaced fuel filter because of miles and everyone says too even though I would be lean and not able to accelerate if it were low pressure. No trouble driving/accelerating
Looked at injector spray patttern under a light, no drips
Manifold vacuum erratic between 15 and 20 inches during rough idle.

Since I can unplug the ESC wire (black and tan) and the issue goes away, it’s as though the ECM is sending the wrong fuel or timing signal based on sensor inputs until some other sensor weighs in and says “whoa there, that’s too much fuel and advance”. Then the correct VE table is referenced and everything goes back to normal. I can’t change the ESC module as it is integral to PCM.

A vacuum leak would be there whether the ESC was connected or not. Same for fuel pressure issue. A bad valve or low compression would also always be there.
Tomorrow night I’m going to check gap on plugs and go after MAP sensor.

What am I missing?
My wife suggested I take it to someone to get it fixed. I’m going to miss her.

 

[DerekTheGreat]

Well, best advice I can tell ya and firing from the hip here would be to find a known good distributor complete with the ignition module and try it in place of yours. I was going to suggest a leak down test with a smoke machine or something but like you said, if the leak was there the truck would run rough with or without the ESC wire being connected but only does it when connected. While not exactly the case you had, mine was similar: I changed the plugs, wires, cap & rotor on my truck. Went to fire it up and it started fine but when it went to try and idle below 1000 rip'ems it died or went to die unless I gave it some throttle. I try and use my timing light to verify the timing and I could see the light was erratic.. Unplug the ESC wire and it's right on at 0*, light nice and consistent. Plug it back in, erratic. So I slapped a new dizzy on there and it's been fine ever since. I know you said you replaced the ICM but like another said, parts now are garbage. My old dizzy was pretty f'in sloppy too, new one was tight. Try a complete dizzy or borrow your friend's and see what happens.

 

[Lattimer]

Between the experience I have had with this Blazer and issues I have had with parts store HEI parts, I don't trust the stuff from Advance or Auto Zone anymore. I've seen at least 4 different times where the new part was bad right out of the box. I rebuilt an HEI that was in my 1970 C-10 with stuff from Advance. Less than 300 miles later and it died, no spark. Ordered a Davis Unified through Summit. 5 years later still work great.

The entire distributor was obviously replaced in my Blazer, it was all shiny and new inside and out. Made in china sticker even. Plus the complete old unit was in a box of crap the PO gave me. Had the exact issue with low idle and miss right after start. Davis Unified Distributor and coil, now no issues.

I think the module was changing the timing way too far and making it stumble. When you unplug the wire, you remove the computer advance and timing stays at zero. Messed up electronics can do very weird things.

Known good distributor would be a good place to start.