350 stumble/stall problem

VT regal · August 8, 2006

I just bought a 1993 Chevy C1500 350 V8 pickup with 130,000 miles. The engine has approx. 70,000 miles. The previous owner was short on cash and I got a great deal, but the truck has some minor problems.

1. It has a slight hesitation in light throttle situations, and feels sluggish when throttle is wide.

2. Occationally it will stall, or start to stall when feathering the throttle, or when coming down to idle speed from a higher RPMs.

3. It grinds when I switch from 4WD Low into 4WD High or 2WD. It does this weather im in N or D. Also, it does not grind when going into 4WD L, only when shifting out. Is the grinding normal? I have no experience with 4WD vehicles.

I will be doing a tuneup here in the next few days. The plugs/cap/rotor look nearly new. The plug wires are in bad shape and will be replaced along with fuel filter and air cleaner. I doubt that will fix the problems though. Can the timing or idle speed be adjusted on this engine, or is it all electronic?

Any sudgestions?

Daubs · August 8, 2006

I believe all GM's from 87 on up have EFI. My first move would be to take to dealer and get that truck on the diagnosis computer. See what's happening. Hesitation and stall may mean injector replacement. I think timing and engine management is all computer controlled...for emissions reasons (and so you and I can't work on our trucks...whole other thread topic).

Did PO have any maintenance records? Plugs/cap/rotor/wires would be good to replace, especially at 130k miles. Might want to replace all fluids also -- tranny, t-case, differentials, etc.

VT regal · August 8, 2006

I believe all GM's from 87 on up have EFI. My first move would be to take to dealer and get that truck on the diagnosis computer. See what's happening. Hesitation and stall may mean injector replacement. I think timing and engine management is all computer controlled...for emissions reasons (and so you and I can't work on our trucks...whole other thread topic).

Did PO have any maintenance records? Plugs/cap/rotor/wires would be good to replace, especially at 130k miles. Might want to replace all fluids also -- tranny, t-case, differentials, etc.

My tuneup didnt help, as predicted. Im thinking injectors as well, at this point.

no matenence records, previos owner replaced the engine a few months ago because of a spun bearing due to coolant in oil from bad headgasket. the junkyard motor in there now was out of a 70,000 mile crashed 95 suburban. thats all I know. the previous owner rarely drove the truck and didnt try to address the stalling/stumbling problem.

mikystyle · August 9, 2006

Sounds like either plugged injectors or a bad fuel pump. Might wanna check out that pump too.

05AZgmc · August 9, 2006

check fuel filter too. minght want to get a fuel pressure gauge set and check the pressure, 30 to 40 psi i think should be good. is bad fuel pressue could be pump or restriction caused by fuel filter, could make it seem like an injector problem cause your injectors arnt gettin enough fuel, if you have good pressure, its probably injectors. you can take the spark plugs out to see if they are really white, if so its running lean which can also indicate a fuel problem, i would test pressure first tho. if this dont work repost and say somthing, ill look through my driveablity school books and see anyother reasons for this to be happening.

VT regal · August 9, 2006

check fuel filter too. minght want to get a fuel pressure gauge set and check the pressure, 30 to 40 psi i think should be good. is bad fuel pressue could be pump or restriction caused by fuel filter, could make it seem like an injector problem cause your injectors arnt gettin enough fuel, if you have good pressure, its probably injectors. you can take the spark plugs out to see if they are really white, if so its running lean which can also indicate a fuel problem, i would test pressure first tho. if this dont work repost and say somthing, ill look through my driveablity school books and see anyother reasons for this to be happening.

<{POST_SNAPBACK}>

Thanks for the sudgestions. Today I replaced the fuel filter so we can rule that out as a problem. I looked at the injectors firing with the engine on, and they looked to be atomizing the fuel correctly (no dripping or innapropriate spraying, but its kinda hard to tell). I checked the EGR passages for blockage and the diaphram for tears (didnt think that was it, but checking was easy and free). The fuel pump does not make any strange noises, I cannot even hear it while in the cab. I also ran a can of seafoam through the brakebooster vacume hose, and no drivability change. The plugs look perfect (tip is black but no buildup of carbon, oil, or gas, porcelain is a medium brown color.)

I think I will take it to my mechanic to do the fuel pump diagnosis.

I drove it for quite awile today, and the problem now only presents itself when feathering the throttle at very low speeds. For example, when pulling out of my driveway, i give it very little gas to get up the hill, and it will almost stall. If I give it anymore gas than that, it will not stumble at all, but that will be too much power and I will spin the wheels in the gravel. The RPMs do tend to fluctuate while driveing more than they should, but that does not effect drivability. I have not gotten it to stall since I have started working on it. It did stall a few times when I first picked up the truck, so mabey my tuneup and other matenance has lessened the problem but did not solve it.

EDIT

In addition, something else has popped up that I am concerned about. When I got the truck, it would not warm up to operating tempurature. I checked the tstat, and sure enough someone had taken it out. I put a stock 195* in, and now there is a slow, heavy knocking sound when the engine has reached operating temp. it is not loud, but I can clearly hear it when outside the truck. It shows up at idle, and follows engine speed. I didnt notice it before because it doesnt start the knocking untill it hits about 180*+.

m1a1a2 · February 9, 2007

You might need to replace your distiributor. There is a shaft off the bottom of the distributor that tends to wear out. As it wears the timing of the engine becomes episodically erratic. The first indication is stalling for no apparent reason. Ultimately the vehicle will with no apparent warning begin missing and bucking as the engine misses. Shut it down an it will restart with no issues. Then you will have another episode, and another, and another...then the shaft on the distriutor will disentegrate and your mechanic will finally figure it out...I am on the 3rd distributior in my Rally STC van with a 350 with TBI. The current engine is a crate engine replacement. Just had to replace the distributor with 60K on it. Some one else with a contemporary suburban told me he had the same problem. There will be no computer code for the mechanic to trouble shoot. I am told that if you shine a timing light on the mark for a long time you may see the timing "jump".