Engine Idles Unevenly and Almost Stalls When Cold

[cc333]

1994 GMC Sierra K2500 w/ 7.4L.

It had been serviced thoroughly a couple years ago with new distributor, new plugs and wires, and a bunch of other things I don't remember anymore.

It runs well once it's warmed up, but on a cold start, it will idle high for a bit like normal, and then when it drops down to low idle kind of suddenly (is that normal?), it starts fluctuating and getting close to stalling, and then it catches itself and revs up again (presumably this is the IAC doing its job).

It does this for a few minutes, and then it stabilizes once it gets up to normal operating temps, at which point it's fine.

It seems to do this in any weather, but it's more pronounced when it's cold (which, in California, isn't saying much ).

The ECU died back in early 2020, at which time I bought a replacement (I transferred the CALPAK, which seems to work at least as well as it did before the ECU conked out, so I assume it's still good?). Could this have anything to do with it? I seem to remember that that was about when the weird idling on cold starts began, but I'm not totally sure. Could the replacement ECU be faulty somehow?

A possible clue to that effect is that the SES indicator will sometimes light up briefly during the rough idling episodes (it will go out when the idle evens out). I don't know the code.

Thanks!

c

 

[movietvet]

Unplug the coolant temp sensor, by the thermostat housing, and see what it does. That sensor tells the computer the engine temperature to help with open and closed loop operations. Is there any CEL? Also, what does it do if you attempt to drive it when doing this?

 

[cc333]

Unplug the coolant temp sensor, by the thermostat housing, and see what it does.

OK, I'll try this out in the next day or two, now that the weather is finally clearing up a bit. But first a quick question (please forgive my ignorance here!): where is the thermostat housing, exactly? Is it where the upper radiator hose connects to the engine?

Is there any CEL?

Yes, it does sometimes come on briefly while the idle is weird, and then it will go off once everything warms up and the idle stabilizes.

Also, what does it do if you attempt to drive it when doing this?

More or less the same thing, except it will sometimes stall if I coast at low speeds (under 10 MPH). Otherwise, it will accelerate and drive normally.

c

 

[movietvet]

Yes, thermostat is at end of upper radiator hose at engine. Need to attach at least a code reader and see what code(s) is in there. You can drive to a parts store and they will likely pull code(s) for free. Write them all down, at least the number, but if can get the definition on the code reader that helps too. They can also just clear the code(s) after getting all of them.

 

[cc333]

OK, thanks!

Since this is a TBI truck, it is OBD I based. Will parts shops have the tools to scan it? I'm pretty sure the local dealer would, as I took it to them a couple years ago for its major service, but I'd like to avoid involving them for this specific thing if I can, at least for the diagnostics, because dealers cost $$$ even for that, and it seems like a relatively easy thing that I can do myself to reduce costs, even if only by a little bit.

c

 

[movietvet]

OK, thanks!

Since this is a TBI truck, it is OBD I based. Will parts shops have the tools to scan it? I'm pretty sure the local dealer would, as I took it to them a couple years ago for its major service, but I'd like to avoid involving them for this specific thing if I can, at least for the diagnostics, because dealers cost $$$ even for that, and it seems like a relatively easy thing that I can do myself to reduce costs, even if only by a little bit.

c

You can do it yourself without a scanner/code reader. Just pulling codes that is. Look at you tube but all you need is a paper clip and pen/paper. Look at your connector, under the dash. Put the clip in the top two spots on the top row right. A and B circuit. KOEO=Key On Engine Off. Watch Service Engine Soon light. Should start to get flashes and should start with code 12. One flash and then 2 flashes after. It does that three times and then codes start. Count the number of flashes and each code will do it 3 times. Then code 12 to finish it out and tell you it is done.

 

[cc333]

@movietvet Ah, yes, the "paperclip trick".

I've heard about it, but I've never actually tried it. I'll see what I can do and report back if I find any codes.

c

 

[Sabinoerc]

, it starts fluctuating and getting close to stalling, and then it catches itself and revs up again (presumably this is the IAC doing its job).

can be a lot of things, hard to say without scanner or codes. That said, I had similar once which was low/marginal fuel pressure. No codes, hunted forever and finally downloaded TunerPro and invested in learning how to look at data. I found that the save from a near stall was an idle enrichment mode which would kick in and rev the engine up/save the stall. It would then turn off and idle would slowly drop until a near stall and repeat. Only happened in open loop. Ran and idled “ok” once into closed loop.

There is no feedback to PCM on fuel pressure so no codes directly pointing at fuel pressure.

Since then, I recommend checking fuel pressure whenever any drivability or idle problem pops up without obvious cause. That, along with checking all the grounds.

I think seeing how it runs without CTS sensor plugged in is interesting. It would keep you in open loop. If problem was after getting into closed loop it might help narrow it down. However, there is easier way to check whether problem is in closed loop or open. There are two pins you can short when running on the ALDL connector to tell if it’s in closed loop or not. I’d check that to see if problem is only in open loop and goes away once in closed. I’d guess that’s the case. Google “GM ALDL field service mode” to find info on how to check that.

good luck!

 

[Road Trip]

can be a lot of things, hard to say without scanner or codes. That said, I had similar once which was low/marginal fuel pressure. No codes, hunted forever and finally downloaded TunerPro and invested in learning how to look at data. I found that the save from a near stall was an idle enrichment mode which would kick in and rev the engine up/save the stall. It would then turn off and idle would slowly drop until a near stall and repeat. Only happened in open loop. Ran and idled “ok” once into closed loop.

There is no feedback to PCM on fuel pressure so no codes directly pointing at fuel pressure.

Since then, I recommend checking fuel pressure whenever any drivability or idle problem pops up without obvious cause. That, along with checking all the grounds.

Sabinoerc,

Your experience is a great example of why giving oneself the gift of live data allows the
GMT400 owner to successfully troubleshoot beyond the real estate directly monitored by the Check Engine
light & associated codes. (ie: No monitoring/reporting of fuel pressure, so we end up with the scenario
where the engine is a victim of a marginal/unmonitored subsystem supporting it.)

It's so much more affordable in the long run to gather data and replace just the root cause vs loading
the parts cannon and A) actually fixing the problem as one of the parts swapped, but at the same time
B) a new bad part installed elsewhere as part of the cannon blast simultaneously covers up the aforementioned
fix...and now the owner is beyond confused after it still runs bad after replacing 'everything'. :-(

Thanks to the relaxed QA these daze, I think this describes a fair number of situations where folks are
joining the forum and asking for troubleshooting assistance after swapping it all out & no joy. (!)

I think seeing how it runs without CTS sensor plugged in is interesting. It would keep you in open loop. If problem was after getting into closed loop it might help narrow it down. However, there is easier way to check whether problem is in closed loop or open. There are two pins you can short when running on the ALDL connector to tell if it’s in closed loop or not. I’d check that to see if problem is only in open loop and goes away once in closed. I’d guess that’s the case. Google “GM ALDL field service mode” to find info on how to check that.

good luck!

And I agree that knowing whether or not the system is running Open Loop vs Closed Loop can
provide a valuable hint of where to look further, especially when the engine isn't running right
but isn't setting the Check Engine light. (!) Here's what the '94 FSM had to say about "Field Service Mode"
and what the flashing light pattern means when in this diagnostic mode:

You must be registered for see images attach

If I owned a TBI GMT400 I think I'd get a matching plug and MacGyver in a small out of the way switch
in the dash. (Even cleaner, install/repurpose a factory switch for an option I don't have. {foglights, etc} )

This way I could easily monitor how my vehicle is going into/out of Closed Loop mode. At least enough
to become familiar with the 'known good' behavior pattern so that I can use this for comparison purposes
if my faithful traveling companion starts to falter down the road.

By the way, I don't have the pinout for the ALDL (Assembly Line Diagnostic Link) but this can be easily
found in the '94 FSM also.

EDIT: I found and included the pinout for this underdash diagnostic connector in reply #11 below.

NOTE: If you ground the diagnostic pin with KOEO (Key On Engine OFF) then you get the
flashing error codes. If you want to go into 'Field Service' mode, this is when you ground the same pin
after the engine is running.

Good stuff. I think this is a great no-cost diagnostic cheat for any TBI owners reading this.

 

[movietvet]

@movietvet Ah, yes, the "paperclip trick".

I've heard about it, but I've never actually tried it. I'll see what I can do and report back if I find any codes.

c

You can also use a piece of wire, insulated or not. If insulated, strip the insulation back about 1/2" on each end and then bend the wire in shape of a "U" and insert it in just like you would have if used a paper clip. Then turn the key to KOEO and watch that Service Engine Soon light, on the dash, and start counting. Just remember, all flashes of codes will be in groups of 3. When you see the first 12 at the end of codes, you don't have to wait for it to flash code 12 three times. When you see the 12, you are done.