'96 4.3L S-10, misfires or hesitates at constant speed

[bbv00]

The vehicle in question is actually my fathers but he is clueless so I am trying to help him out. His truck starts, runs, idles fine but once on the highway at constant speed, the engine misfires or hesitates causing a small buck or jerk but continues on down the road. The SES light is on. He took it to O'Reilly and they put some kind of computer on it and (I think) it said misfire in a cylinder. He replaced one of four O2 sensors, pickup in the distributor (not sure what this is called), wires, and plugs with no cure. This problem is very strange as again, it only does it a constant speed; truck runs fine at idle, accerleration, deceleration, startup, etc. There may be a slight sulfur smell during idle and there is a decrease in gas mileage. Anyone have an idea? I am hoping this is a common problem, but haven't found any. Any help would be greatly appreciated.

[Cam1959]

I might have thought something to do with ignition timing vacuum advance not backing off at constant low rpm but it idles fine.

You say on highway constant speed. Could this be when it tries to go into overdrive high gear, but if any brake drag or road incline, load on motor, it will jerk in and out of that high gear ?

If this happens at city constant speed, my next guess is fuel/air mixture. Idle ok since it gets richer mix. So maybe not enough fuel getting in at low rpm highway cruise ?

[bbv00]

it definitely has nothing to do with the tranny. It never shifts or tries too or anything. it is almost as if you shut the engine down for a second, and then fire it right back up. but it bucks or jerks, almost as if the engine backfires or something. however, there is no noticeable noise. only does it while goind down the highway at constant speed. SES light is on all the time. Slight sulfur smell in the exhaust and gas mileage has decreased. thanks for your reply.

[RahX]

Sounds like a slipping torque converter clutch. Get the tranny flushed and use Mercon V/ATF+4. Its a bit expensive but it takes care of a slipping TCC. You can also get a friction modifier additive if you dont want to pay the extra money per quart and use dex/merc 3.

[drdd]

sulfur smell is a sign of a clogged catalytic converter. Bad cat would also cause the decreasing gas mileage.

Originally Posted by bbv00

it definitely has nothing to do with the tranny. It never shifts or tries too or anything. it is almost as if you shut the engine down for a second, and then fire it right back up. but it bucks or jerks, almost as if the engine backfires or something. however, there is no noticeable noise. only does it while goind down the highway at constant speed. SES light is on all the time. Slight sulfur smell in the exhaust and gas mileage has decreased. thanks for your reply.

[dewaynep]

I just went through this for the past 2 days with a 96 Astro with the 4.3l. Do yourself a favor and go to the Chevy dealer and get a genuine AC/Delco distributor cap and rotor. Do not go anywhere but the dealer. I chased this problem for 2 solid days following the advice of several people on several boards checking this and that. Luckily I had the equipment to check the items that others had problems with. I never thought it was my cap and rotor because I had just purchased the best one Autozone had at a cost of $57. I had also changed plugs and wires at the same time with no change to the misfire at cruising speeds. Well, after eliminating every other possible cause I was left with the possibility the cap and rotor were bad. I drove my van to the dealer to purchase the cap and rotor and installed them in the parking lot. I have not had a misfire since. That was the best $54 I ever spent. Good luck.

By the way, the code should have been p0300 general misfire, meaning it is not just one cylinder. The pickup in the distributor is just the cmp sensor (camshaft position sensor). That sensor has nothing to do with timing or ignition. The only thing it does is to correlate with the Crankshaft position sensor to tell the pcm what cylinder misfired. The truck will run fine without that sensor connected, but you will get a code for a failed cmp sensor high voltage.

[bbv00]

Thanks to all for your advice. I will let you know if any of the suggestions cures the problem. Thanks.

[c1971]

Did you check EGR valve?

[sourgrout]

I have the EXACT same thing happening with my 1990 2.8L. When I disable the egr valve it goes away, however this makes the engine light come on. I have done a full tune up, except fuel injectors which is my next step. Oh yeah I replaced the egr valve and no difference, it feels like it shuts down for a split second then goes again at lower cruising speed like 35 mph and will keep doing it until I give it a little more gas. Good luck.Let me know what cures it!!
Oh by the way its not the transmission these people dont know what the hell there talking about, (sorry guys) and acdelco cap and rotor wont do anything either. From what Ive been able to determine it is fuel/air mixture. Clean the egr out( all the carbon) and try that, then look for map sensor, engine control sensor, O2 (already did that), fuel pressure, fuel filter, fuel injectors. Autozone has an online repair manual that will help you locate and test these parts, pretty simple procedure if you have a multimeter.

[dewaynep]

Sourgrout: this is a different year and engine combination than you have. The above remedies are valid for the 4.3l engine as stated in the problem. You may want to post a seperate question so people who have experience with YOUR engine can help you. To offer off-hand remarks (quote:"they don't know what the hell they are talking about") about people who are trying to help is irresponsible on your part, especially when you have a completely different drivetrain.

[chevyLex5150]

i have the same vehicle with the same problem 1996 s10 vortech 4.3 automatic vin.x. exact problem. have p0340 dtc (cam sensor). i have brand new acdelco cap and rotor from dealer. this problem is all over the internet forrums so must be common, but i have not found the fix yet. ive been driving my truck like this for 2 years!!! never actually died or left me stranded just extremely anoying not to mention embarrassing when i have company. im so frustrated. def. not trans related, u can drop a gear when problem is occurring with no change. ive pulled distributor twice to check the gear(doesnt happen all the time so i doubted that anyway), ive checked waveform of cam sensor with snap on vantage and it seems fine. my power is fine so i doubted a clogged cat. ive diconnected egr and it doesnt go away, plugs and wires are 6 mo. old and ive checked them by spraying water on them to look for arching. nothing. sometimes it does it really bad(longer) and when it does it will also throw a p0306 misfire cyl. 6 in addition to the p0340. wont do it while im checking it so everything reads normal when testing.im pulling my hair out!! has anyone fixed this???

[FishFind]

sulfur=Bad cat. take out the 02 sensor before the cat. Drivers side is easy to remove and try the highway drive again.

[bigmak5996]

so i have a 1995 s-10 with a 4.3. my truck is doing it every day for the last 3 months. i keep thinking it`s a spark problem as mentioned before. it could possibly be a fuel problem but i keep doubting it. i have read the other post here and i am still waiting to see if you have had this problem fixed yet. or if there is any new suggestions yet to try. i have gotten new fuel filter,plugs,wires,button,and cap. {not however the expensive cap from the dealership}

[FishFind]

you need to check for DTC codes. This year is ODB1.. Also check for wiggle in the actual distributor.

[bigmak5996]

ty fish i will do that this weekend will let you know how it works out