Rough running when cold

[austinado16]

The 'burb is running rough at idle, when cold, and it's progressively getting worse. It's fine once warmed up. A quick check with a timing light, and it's got spark at all 8 plug wires.

Wires are brand new AC/Delco.
Plugs are brand new AC's.
Cap and rotor are a year old, and whatever NAPA sells.
Distributor assembly is about a year old, from NAPA.
Coil is an MSD. Original coil doesn't improve the condition.
PCV valve is new, from NAPA
EGR valve is about 2 yrs old (from NAPA)
Intake manifold is Edelbrock, installed a year or so ago. No external leaks when sprayed with carb cleaner or starting fluid, etc.

Runs smooth and nice once warmed up, including idle, although the idle seems too low, especially with a/c on.

My guess would be that it's too lean when cold. How do the TBI's manage cold mixture enrichment and how do they control idle?

Any idea what the issue is? Thanks!

 

[Jims86]

The 'burb is running rough at idle, when cold, and it's progressively getting worse. It's fine once warmed up. A quick check with a timing light, and it's got spark at all 8 plug wires.

Wires are brand new AC/Delco.
Plugs are brand new AC's.
Cap and rotor are a year old, and whatever NAPA sells.
Distributor assembly is about a year old, from NAPA.
Coil is an MSD. Original coil doesn't improve the condition.
PCV valve is new, from NAPA
EGR vavle is about 2 yrs old (from NAPA)
Intake manifold is Edelbrock, installed a year or so ago. No external leaks when sprayed with carb cleaner or starting fluid, etc.

Runs smooth and nice once warmed up, including idle, although the idle seems too low, especially with a/c on.

My guess would be that it's too lean when cold. How do the TBI's manage cold mixture enrichment and how do they control idle?

Any idea what the issue is? Thanks!

Coolant temp sensor, and vac signal to the map sensor are the two main sensors taken into consideration by the ECM during open loop, o2 is ignored. Idle air is controlled by the IAC.
Vac leaks on a speed density system controlled engine will usually be a higher idle, but fairly smooth, unlike a Mass Air Flow system, or carburetor.
2 things happen with a vac leak ecm wise on Speed Density.
1. There is a slight decrease in vac signal to MAP sensor, and 2. the o2 sensor reads the extra oxygen....both signalling the ECM to increase the injector duty cycle to add fuel to accomodate the condition.
Do you have a scan tool to read parameters?
Any codes stored in ecm?

Compression check?

 

[89Suburban]

 

[austinado16]

Coolant temp sensor, and vac signal to the map sensor are the two main sensors taken into consideration by the ECM during open loop, o2 is ignored. Idle air is controlled by the IAC.
Vac leaks on a speed density system controlled engine will usually be a higher idle, but fairly smooth, unlike a Mass Air Flow system, or carburetor.
2 things happen with a vac leak ecm wise on Speed Density.
1. There is a slight decrease in vac signal to MAP sensor, and 2. the o2 sensor reads the extra oxygen....both signalling the ECM to increase the injector duty cycle to add fuel to accomodate the condition.
Do you have a scan tool to read parameters?
Any codes stored in ecm?

Compression check?

Thanks!

Bare with me. I don't have a factory service manual.

-Where's the IAC located?
-Where's the coolant temp sensor that feeds the ECM?
-Where's the vac line that feeds the ECM?
-Where's the diagnostic test port for reading the codes?

I don't have a scan tool for GM stuff, but I've got the RossTech program for Audi/VW (what I used to work on) with both OBD1 and OBD2 cable connections. It may be able to perform a generic read of codes, but I doubt it would read parameters.

-Which scan tool works best on this 1990 GM TBI 'burb?

Haven't done a compression test in a couple years. I can, but I don't think compression is an issue, because this a cold running condition and goes away once warmed up. If it were happening cold and hot, I'd be thinking it were a mechanical problem...like flat cam lobes, lifter or rocker arm issue, burned valve, etc. But it runs great at all other times, and pulls like a freight train towing our 2,000lb pop-up camper 5hrs each way up into the mountains (just got back from our 2nd trip in 2 weeks).

I do have a vacuum leak somewhere in the HVAC system, because the recirculation door will open when climbing a steep grade. I've traced it as far as the diaphragms on the heater box under the dash, but didn't continue because I didn't feel like taking the heater and dash apart further. If that could be the cause of the poor cold running, I could plug that vac line and see if the cold rough running improves.

 

[89Suburban]

-Where's the IAC located? Facing the motor it is on the left rear corner of the throttlebody, screws in on an angle behind the Throttle Position Sensor.
-Where's the coolant temp sensor that feeds the ECM? That is the sensor to the left of the thermostat housing.
-Where's the vac line that feeds the ECM? Comes out of the front of the throttle body and goes across to the MAP sensor, square black box next to the TPS. Do not confuse with the EGR control valve which is mounted piggyback to it.
-Where's the diagnostic test port for reading the codes? Above the parking brake mounted to the bottom of the dash. You can use a piece of wire to get codes read using the check engine light - http://www.gmsquarebody.com/forum/showthread.php?t=6844
.

 

[austinado16]

Thanks so much for posting all that.

Do the IAC valves get funky inside and stop working properly.......or simply fail to function properly due to time and mileage? Or is it pretty rare to have one die? Reason I ask is that the idle seems kinda low once warmed up, especially with the a/c on. Low, and will have just a slight surge when sitting at a traffic light. Can the IAC be cleaned out....like spray it out with carb cleaner and WD-40?

Been spinning wrenches on German stuff, both professionally, and as hobby, for decades, but have done very little to my sole GM product.

 

[Jims86]

Thanks so much for posting all that.

Do the IAC valves get funky inside and stop working properly.......or simply fail to function properly due to time and mileage? Or is it pretty rare to have one die? Reason I ask is that the idle seems kinda low once warmed up, especially with the a/c on. Low, and will have just a slight surge when sitting at a traffic light. Can the IAC be cleaned out....like spray it out with carb cleaner and WD-40?

Been spinning wrenches on German stuff, both professionally, and as hobby, for decades, but have done very little to my sole GM product.

John, yer a ******* Rock Star!

IAC valves do get carboned up sometimes, and will affect idle quality.
It can be removed and cleaned...it screws out, but just snug it like a carb nut when you re install it. Another thing to see if its functioning, is to: Look down into the throttle body pass side, into the cavity that the IAC points into, you should not see the cone shaped pintle in the port.
jump ports A and B on the diagnostic connector.
turn the key to the on position( dont start) and run back around and look inside the throttle body, and around 30 seconds, you should see the IAC valve sticking into the IAC port.
If not, replace the valve because its not working.
I personally would get a TBI rebuild kit and clean everything up, along with the new IAC.
First thing you should do is the ECM diagnostic check, even if you have no light, as it could point you in the direction of the problem.
I will let you know right now, that these ECM's have had problems with injector drivers going south, and have made factory trained GM techs commit themselves to the insane asylum trying to track the problem down....but lets do the normal checks first.

 

[austinado16]

Thanks for the detailed info. That's what I'm after. I'll report back as soon as I have a chance to check codes, and watch function, and then clean or replace the IAC.

This engine has just rolled 182k, and for the most part, is original and never been touched really other than the Edelbrock intake I installed, and the distributor. It's probably had a waterpump at some point, but you can't tell by looking at it. It runs amazing, and even with 3.42's, IMO, the 'burb gets with the program if I put my foot into it. I never drive it that way though, but I have rolled out onto the freeway down a long onramp and let 'er spool up. Pretty amazing. At 55, where we typically drive it, it's only twisting about 1,700rpm on the tach in OD with TC locked.

However, the cold running situation is not giving me a warm fuzzy, so it's time to dig in.

 

[Jims86]

Thanks for the detailed info. That's what I'm after. I'll report back as soon as I have a chance to check codes, and watch function, and then clean or replace the IAC.

This engine has just rolled 182k, and for the most part, is original and never been touched really other than the Edelbrock intake I installed, and the distributor. It's probably had a waterpump at some point, but you can't tell by looking at it. It runs amazing, and even with 3.42's, IMO, the 'burb gets with the program if I put my foot into it. I never drive it that way though, but I have rolled out onto the freeway down a long onramp and let 'er spool up. Pretty amazing. At 55, where we typically drive it, it's only twisting about 1,700rpm on the tach in OD with TC locked.

However, the cold running situation is not giving me a warm fuzzy, so it's time to dig in.

It could even be a head gasket issue that seals when everything warms up....but lets go one step at a time.

 

[austinado16]

I've never seen a self sealing head gasket leak, and I've seen and repaired hundreds. Plus, where we tow, the engine would have grenaded already, if there was a head gasket issue.

We climb a 30mi long grade, from about 200' to 7,500', in 80-90*F temps, towing a 2,000lb pop-up camper. It's 2 lane, and twisty, so it winds up being a slow 35mph grind with the radiator fan clutch engaged, and typically in 2nd gear. Then we drop down an even steeper grade to 3,500, in 2nd, and then climb back to 4,500'. And 3 or 4 days later, do it all backwards, starting with the 25mi 7% haul back up to 7,500', in 2nd. This is our 7th season and I have no idea why it hasn't chucked a rod by now.

 

[Jims86]

I've never seen a self sealing head gasket leak, and I've seen and repaired hundreds. Plus, where we tow, the engine would have grenaded already, if there was a head gasket issue.

We climb a 30mi long grade, from about 200' to 7,500', in 80-90*F temps, towing a 2,000lb pop-up camper. It's 2 lane, and twisty, so it winds up being a slow 35mph grind with the radiator fan clutch engaged, and typically in 2nd gear. Then we drop down an even steeper grade to 3,500, in 2nd, and then climb back to 4,500'. And 3 or 4 days later, do it all backwards, starting with the 25mi 7% haul back up to 7,500', in 2nd. This is our 7th season and I have no idea why it hasn't chucked a rod by now.

Damn....where are you again?

 

[austinado16]

Coastal CA. We haul 5hrs to get up into the Sierra's and Kings Canyon National Park on HWY 180.

I've made a few mods to help the 'burb make this, and other hot hauls:
-Transmission is rebuilt with all the latest/greatest upgrade components, a mild shift kit, the big B&M finned aluminum pan that holds 3.5 extra quarts, and a mechanical trans temp gauge bunged into the pan.

-11"x11"x1.5" Tru-Cool (B&M Super Cooler) trans cooler with an original GM 3/4 ton pusher fan.
Early configuration:

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Current configuration:

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-It was already equiped with the big engine oil cooler and P/S cooler.

-Last year I installed the much deeper 3/4 ton radiator....but I'm still looking for an original wider version of that same deep radiator.

-Mechanical oil pressure gauge (surprisingly the factory gauge is accurate!)

-Finned cast aluminum diff cover that holds about 1/2qt extra

-All synthetic oils in engine/trans/both diffs