GMC Stall When Cold

The "Big Bopper" I don't know why I call it that, just came up with this problem. It's my 91 GMC Sierra 1500. 350 with throttle body injection. 2wd auto. Close to 200k on the body, but I put in a rebuilt 350 that now has around 40k on it. I rebuilt the throttle body a while back, not too complicated. I've replaced the intake gaskets once already, old manifold with new engine...

I just put new plugs in it that really helped with a miss it always seems to have at idle. Problem is it stalls now a few times when I start it up. I'm sure it has some kind of temp switch activated system that acts as a fuel enrichment device for cold starting. It used to idle a little higher on startup, indicating whatever cold start device it has was functioning. I replaced the TPS once too. No check engine light coming on.

I don't know if I should think along the lines of fuel pump going bad, fuel pressure regulator, or fuel filter. Or generally shitty gas. It runs fine once it warms up just a little.

I know this is kinda vague, looking for some input.

Dan, start by checking the obvious things. Fuel pressure, vacuum leaks (especially around the base of the TBI), etc etc. I think the fuel pressure should be around 13 psi for a TBI setup, but check with "Mr. Goodwrench" to be sure.

Assuming all of that stuff checks out, one of the few sensors that is used during cold-start operation is the ECT..."engine coolant temperature" sensor. I think it is located right next to the thermostat housing, and should be fairly inexpensive. I'm sure you can test it with a multi-meter, but I have no idea what the values should be.

Clean the throttle body good and check the IAC (Idle air control), it's like a bypass for the throttle body, if clogged, could cause the stalling. Probably a temp sensor like mentioned above.

Does it have a computer plug-in under the dash at all? Sometimes you can stop by the autozone or equal and they can plug in a scanner and find any codes, maybe narrow it down. I have a older ODB scanner, it flashes the check engine light for the codes.

Thanks,
I have a code scanner but it didn't show anything, checked the IAC, took it out to check and clean, don't know how to check it electrically. That IAC is probably not being told to operate. TBI injectors are spraying nice cones. Poking around vac lines etc. O2 sensor is relatively new.. cat is 2 yrs. old. Checked temp sensor at thermostat, no resistance with engine warm.
. Now I see I have an exhaust leak at the manifold/y pipe. I'll have to carefully remove and try to drill out the flange bolts I know are gonna snap off the exhaust manifold . All this because NJ State inspection is due soon. If I take it to the local place they will laugh at me and then break something else. They are theives.

thanks for ideas.

I had the same issue in my '93 Chevy Z71 1500. Never did isolate the problem. I was told it was bad grounds, temp sensor, starter. I got tired of it so I bought a Dodge and gave the Chevy to my friend. It still does it.

I had the same thing with my 97 Explorer, The ICA was stuck, took it off and cleaned it out with carb cleaner. Has worked ok since. It sits alot. Has less than 60,000 on it.

crazy4coal wrote: It sits alot.

Yeah that's part of it with the GMC. It sits a lot these days. Been so rainy and humid too. It's got "sittin' rust" on the exhaust manifolds and rear shackles, u bolts holding the rear on, etc; and now I'm wondering about my axles from reading the other thread Did the rear outer seals a while ago and I think there was measurable play. I've carried some loads with that truck... still want to fix it up though.

I would seek another used truck but I've become so paranoid about the intense dishonesty of human beings that I'm afraid to even look at anything. Especially with all the flooding this summer regionwide!

I don't know about Chevys but if you unplug the IAC valve,the truck should stumble or idle lower, telling you that it is working.

the temp sensor with the yellow and black wires is the item i would check via a scan tool with ability to see real time values.
also the base tps setting. base idle... for mixture enrichment the milisecond injector pulse is raised. timing chain slop will affect the timing, not so much cam timing being a big deal but might have to bump the ignition timing up a smidge if it got a conscributer banition in it still

Some good info on diagnostics
http://www.aa1car.com/
Good luck
Bill

More advice thanks!

The IAC valve disconnect didn't do anything, but I tried it. Might mean it's beat ! In other words it might be dead and that's why the truck starts hard. It screws in, has 4 leads. I dont know if it's solenoid or stepper motor type. I could back it off and see if it increases the idle. I guess it just bleeds air past the butterflys.

I can't diagnose with any sophistocation at all. Not sure if the TPS has any adjustment, I know some of them have rotational adjustment.

One thing I've learned is never tell an auto repair shop you just want to pay for diagnosis. It threatens and insults them at the same time.

The sensor with the red and black wires, right next to the thermostat housing, is cheap to replace. I could pull it but will have to drain antifreeze below its level. Then I could heat it or cool it to see if it is a thermistor-- change resistance with temp. I was looking around Rockauto to see what they have. ( Thinking about my swap parts until the problem goes away method that has never worked before...)

There's also an EGR time delay switch. It might function to prevent EGR from working until warmup. Along those lines, I put my finger on the EGR diaphragm and opened it which roughened the idle considerably. I'll pull the EGR tomorrow and clean it. Maybe that's it. That was replaced at one time- mad expensive for a simple device.

Old distrib is in it, plenty wear on it. I think I put a new gear on it though.
Engine has always had good oil pressure- but might have slack in the timing chain after 35k.



I got the 350 off EBAY! some clandestine machine shop in Philly. It was under a grand for a long block. I put it in at 165k when the old engine was clattering. The poor slob that delivered it couldn't get the gate lift to work. I fixed the gate lift, a straight box truck. It had dual diesel tanks. He hit a rock and knocked the drain plug right out of the right tank. Diesel was pouring out down our lane! I ran inside and cut a piece of closet rod and sanded it to a taper. Ran back out and banged it in like a bung! The guy drove away. I think this is a startup situation. It starts hard now even warm, but runs ok. Engine is clean, no oil leaks.

not sure if you checked/replaced this yet but check the ground strap that goes to the block, i had a 89 burban with a tbi 350 and had some problems like you said and it was a corroded ground strap, added a new 4awg ground and it fixed it.

Keep it simple brother Dan. Might just need cap, rotor and/or wires.

If not, scrape all the carbon out of that stupid, useless EGR .... the EGREGIOUS valve (the BAD meaning).

Thanks for more info. The 350 is a tried and true engine, I don't know when it came out, but when it did, it had a carburetor that you could rebuild, a distributor with a set of points and a vacuum advance and that's it!

I have an extra ground strap to the block, but better check it anyway. Hope to get this problem solved. Coalfire (Eric) is helping me, greatly appreciated.

Now I found the Y pipe off the exh manifolds is shot. I might be able to braze the rust out but I doubt it. gotta take it off one way or the other. I had to take my Dremel tool to the nuts on the three studs comiong off the exhaust manifold. Then I took an impact chisel to the situation. At least the threaded rod i used to rig up the exhaust is soft. Which means when the rod twists off in the manifold flange I should be able to "hog out" the studs; as Smitty puts it.

Rust never sleeps

Right in the middle of all this, getting flying rust in my face and eyes despite my cheap plastic eye protector glasses; my Dremel quit. Took it in to the desk here and was able to take it apart, chuck the armature into the drill press and dress the commutator with 320 and 600 emery paper. Blasted the slide switch with alcohol and compressed air, dried it, put it all back together. Still dead. Then I took the brushes out and pulled the springs to give them more pressure against the commutator. That did the trick. Grabbed the tool to go out, lie on my back in the gravel,, started raining again .

Thanks for all the replies and help, especially Coalfire who took time to talk to me.

Cap, rotor, plug wires, IAC and EGR corrected the problem. It's running pretty good now. I think it was the IAC, I replaced the old one with genuine Delco.


I like to go to NJ State Inspection because it's free. But you do get that 'big brother is watching' feeling...

I heard they are a little more lenient now, for whatever that's worth. They used to have a machine that vibrated the hell out of your front end. Freaking ridiculous.

But I guess it's for the greater good of society, I used to work in an alternator shop in Florida where there was no vehicle inspection. I moved a customer's car once and it hit the fence from bad brakes....

I slapped high heat aluminum paint all over the exhaust system and it's still baking. My tires may be a problem, they're getting close to being baloney skins.