Chevy S10 trouble shooting

Jeep450r

Jeep450r

NAXJA Forum User
Location
Nor*cal
Hey guys. I'm getting ready to sell my S10 and need some help trouble-shooting a problem with it.

Specs:
94 Chevrolet S10
4.3 Vortec CPI V6
Auto trans


I'm experiencing a rough idle once it warms up. Seems like a random misfire and isn't super consistent. No misfire upon initial startup when cold.

When I smogged it, it barely passed the HydroCarbon reading - had 1 HC to spare... Which I look at as a misfire/unburnt fuel issue.


What should I check?

I doubt it's any hard ignition source such as cap/rotor/plugs/wires because it idles perfectly fine when cold.

Lots of what I've been reading points to the EGR valve. So I pulled it off and cleaned the valve (which wasn't sticking and didn't have very much carbon buildup and did an ohm/resistance test on the solenoid which turned out fine. Put it back in and have the same problem.


Could the MAP sensor be an issue?

Bad PCM?

Not sure what to check now, really. I hate electronics.

All ideas will be appreciated. Thanks guys.
 
How is the cat ? O2 sensor? Tps? Coolant trmp sensor ? Clean grounds .
 
No idea how the cat is, nor the O2 sensors. But I don't see those as causing the rough idle, right? I think of those more as helping fight the high HCs, rather than the root cause of the misfire. Is that a wrong assumption?

Unplugging the TPS doesn't change anything. Assuming that when I do so, the computer takes over and replaces the lack of TPS readings with a set factory settings, I doubt the TPS is bad. I should pull it off and test it though.

Coolant temp sensor is bad... Either the sender or the sensor because the gauge barely reads anything once it's warmed up... How would this effect the rough idle though? Makes the computer think it's running rich until it reads warm (which it never actually reads)? I can see that but I'm still confused about how that would effect the rough idle.

Good call on the grounds too. I'll double check all the vacuum lines as well.


Any other ideas? My roommate keeps telling me MAP or PCM but I'm clueless on those.
 
To my knowledge, MAF sensors replaced MAP sensors in 96 on these motors.

The O2 sensor is making more sense to me now. I remember my jeep running/idling like crap when my upstream sensor went out. Is there a way to test those bad boys? I'm really not trying to dump any money into it by throwing parts at it. Just want to sell it so I can move on to other projects.
 
I think you can test them with a multimeter.
 
you have an intake manifold leek . I asume no check engine light is on cause you passed smog. HC is hi O2 sees extra unmetered air and richen up mixter . just my guess good luck
 
I know that the 94 and 95 GM 4.3L vortec engines had major issues with the spider style fuel injectors clogging. I was working at a GM dealer and they were power flushing injectors on the 4.3 on a pretty regular basis. I think they were flushing them using a special canister with cleaner at 60+ psi. Might want to check with a GM shop/dealer and see what they suggest.
 
use a spray water bottle
 
Thanks for the info guys. I'm planning on checking all vacuum and the intake manifold, testing the map, and testing the o2 sensor first. Then I'll move on to fuel
 
Update:

Egr got stuck open (clogged) and threw a code. Cleaned it out and never plugged it back in (on purpose so it wouldn't open and get clogged again) and it ran like a champ (20 miles). Brought it home and let it idle. had the slightest miss at idle after idling for a few minutes. Turned it off and let it sit for a day. Went to start it to drive to work and it had a hard, constant misfire at all times through all Rpms. No power either.

The tach jumps around while driving it, which makes me think it's an electrical issue - pointing to ignition.

Here's what I've done:
Plugs
Wires
Distributor cap and rotor
Ignition coil
Ignition control module

Fuel pressure reads 55psi and leaks down so slow that you can't see the needle move. I think 58 is the low end of the factory spec. I think it's fine seeing that it never cuts out or gets starved of fuel at WOT. Slow leak down makes me think fuel pressure regulator is fine.

Spider injector/lines? It's central port injection. I really don't to pull the intake off to do the $300 injectors.

Something else ignition?

Any ideas will be appreciated. I'm about to throw in the towel and burn it to the ground.
 
Egr. Make a block off plate using part of an aluminum can. Bolt the egr back in place with the block off plate sandwiched between the egr valve and gasket and recheck.

If that doesn't do it run a compression test just to be sure you aren't dumping money into a junk engine.
 



Made the block off plate. Still have the super bad misfire. Gonna check timing next week and then I'll do a compression check. I just feel like it's something simple that I looked over.
 
Got home from work today and went to pull the s10 into the garage to do a compression check and it started up totally fine and idled great. Threw everything back on it and test drove it a few miles and it's never run better. No idea what happened but I'm not complaining.

Any idea if I'll be able to pass smog with this egr block off plate? I doubt it will but it's worth asking I guess.
 
Its doubtful you will pass smog with it on there. If the tech doesn't see it the test machine will most likely register higher then normal NOx readings.

Intermittent running problems are tough to nail down. If it is running ok don't bother doing much more diag on it. Wait until it is acting up and check it right then.
 
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