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


bbv00
04-03-2006, 11:29 AM
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
04-04-2006, 01:36 PM
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
04-04-2006, 03:20 PM
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
04-05-2006, 12:34 AM
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
04-05-2006, 04:04 PM
sulfur smell is a sign of a clogged catalytic converter. Bad cat would also cause the decreasing gas mileage.


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
04-06-2006, 11:08 AM
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
04-07-2006, 10:12 AM
Thanks to all for your advice. I will let you know if any of the suggestions cures the problem. Thanks.

c1971
04-14-2006, 06:45 AM
Did you check EGR valve?

sourgrout
04-14-2006, 08:48 PM
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
04-16-2006, 07:15 PM
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.:2cents:

chevyLex5150
03-29-2010, 10:35 AM
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
04-04-2010, 09:41 PM
sulfur=Bad cat. take out the 02 sensor before the cat. Drivers side is easy to remove and try the highway drive again.

bigmak5996
05-14-2012, 09:00 PM
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
05-14-2012, 09:07 PM
you need to check for DTC codes. This year is ODB1.. Also check for wiggle in the actual distributor.

bigmak5996
05-14-2012, 09:29 PM
ty fish i will do that this weekend will let you know how it works out

j cAT
05-15-2012, 09:06 AM
ty fish i will do that this weekend will let you know how it works out

getting the codes is a good idea before spending money on other items. what you did so far is just routine maintenace. these items need periodic replacing before you have problems. on replacing these items inspection of the wire resistance plug color cut open fuel filter checking for restrictions etc arcing inside the cap. all this helps you to determine whats going on.

items that can be defective are the oil pressure fuel switch and the ICM. with the age of this vehicle these components are in most cases not working correctly. often these items are ignored as the engine performance slowly suffers.

theo715
10-16-2012, 09:50 AM
The same problem on my 98 4.3L s10 the code is random missfire and Ive changed to a AC/Delco, new wires and plugs and still does this. No cat smells or anything. So im thinking maybe a bad enjector on a cylinder or something?

copdills
10-13-2013, 08:48 AM
my 98 s10 is doing the same thing random misfires when checked runs great as long as you give it gas , but I have noticed when I fill my gas tank up it runs great until I get back to 3/4 full and it starts missing again at constant highway speed and the ses light comes back on , I can drive 30-50 more miles light ses light goes out runs good for another 50 miles or so and starts missing again at constant highway speed , Thanks for any replies

Blue Bowtie
10-13-2013, 09:45 PM
Have you retrieved the DTCs? That will remove a lot of guesswork.

There is a chance the EVAP system has a problem. My '96 Sonoma also generated P0440 codes with a partial tank. Replacement of the purge valve on the EVAP canister and several vacuum lines solved it.

21sprint
11-06-2013, 10:08 PM
Got a 1997 with same problem. !st off changed wires, plugs, cap and rotor with AC DELCO parts. Still same problem. Checked fuel pressure with key on 65 lbs and driving 55lbs. Tried a rebuilt Distributor, another set of wires, plugs and cap and rotor. plus a new EGR valve, MAP senor and MAF sensor. The truck seems to run better with EGR unhooked. I replaced intake gasket(it was bad) and thought I had it, no. I just replace both upstream 02 sensors now and no help. This truck sort of falls on it's face and bucks and jerks, only codes in missfire and EGR pintle.I watch my computor and it shows missfires on every cyl. but never says if one cyl. is the problem. I wonder if the pump is not really putting the volume to injectors. I've went way to far on this truck to stop now. I can't beleive it's the pump though, it starts fast and holds fuel pressure for 30 minutes after I turn key off.

copdills
11-08-2013, 04:01 PM
I can fill mine up with gas and it doesn't miss for about 80-100 miles, I had it tuned up 7 months ago with orginal parts my mechanic told me it might be my gas gap, replaced that no luck if I fine my problem I will let you know , please do the same for me with yours , I hope you het your truck running right , I bought mine new and would like to get it fixed
thanks Randy Dills copdills

Tech II
11-08-2013, 08:30 PM
It's not really rocket science.....

If you do not have access to a scan tool that can pinpoint which cylinder is misfiring, first thing is check spark output at the end of each ignition wire with a spark tester......

An old trick is to cut small pieces of washer fluid hose, and insert them between the wire and the distributor......this hose is high in carbon.......now with the car running, take a test light, ground it, and then touch each piece coming out of the distributor.....this kills a cylinder......now, if you kill a cylinder, and rpms drop you know that circuit is firing.....if there is no difference, then you've found your bad cylinder......if you have good spark at the end of the line with a spark tester, and compression is good, it's leading towards an injector....those spider assemblies were known to have stuck poppet valves, that caused misfires......

With CPI, there is one injector and the whole assembly has to be replaced....the next generation was CSFI, with 6 individual injectors, so the whole unit did not have to be replaced, only the bad injector......Second generation uses a MAF sensor......make absolutely sure there are no air leaks between the MAF and the intake.....

21sprint
11-09-2013, 12:14 AM
I'm using a old Snap On computor and mine shows missfires jumping around, new Spider injection and new rebuilt distribtor didn't help, This truck feels like its losing power when you hold it down and when you back off a little it just starts surging badly if you hold it in that position.

Tech II
11-09-2013, 01:14 PM
Cat could be plugged....

j cAT
11-09-2013, 01:58 PM
I'm using a old Snap On computor and mine shows missfires jumping around, new Spider injection and new rebuilt distribtor didn't help, This truck feels like its losing power when you hold it down and when you back off a little it just starts surging badly if you hold it in that position.

remove the upstream O2 sensor .if with the hole open and the engine is running better then the exhaust is restricted.

21sprint
11-12-2013, 07:43 AM
remove the upstream O2 sensor .if with the hole open and the engine is running better then the exhaust is restricted.

Cat was stopped up. I've never had a truck nor car act like this one, but that was the problem.Thanks

j cAT
11-13-2013, 02:07 PM
Cat was stopped up. I've never had a truck nor car act like this one, but that was the problem.Thanks

hopefully the unburned fuel issue was fixed. this is what normally cause the cat converter to get damaged/plugged.

the O2 sensor removal trick is a well known fast way to determine the issues you have .. your vehicles operation is 90% of what the restricted exhaust would do ..

good luck ...

snakeongrass
12-11-2013, 12:30 PM
i had this problem (jerking on freeway at constant speed). if there is no engine light on..the problem is more then likely your crank shaftsensor.rightunder harmonic balancer.. this fixed my problem for two years. i am having this same problem two years later . this timeit isnot the sensor. inow have a transmission code..it can be either one ..snakeongrass:loser:

snakeongrass
12-12-2013, 07:27 PM
hey i just found this .. i think its my spacer for crankshaft sensor this time ..https://www.youtube.com/watch?v=KLC0E7N47Lw
if you dont get link . there is a youtube video talking about needing a spacer for new crankshaft sensors.. i am 99% sure you guys have a crankshaft sensor out.i have replaced one years ago for same symptom in the same truck .. this guy says that chevrolet has made a notic about needing shims. snakeongrass:loser:

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