Misfire in my 91 chevy truck 5.7

[morepwr]

OK guys, I am out of ideas here on this truck. It is a 91 chevy truck with the 350 (5.7) with TBI. The truck has 190K miles on it and runs fine other than a misfire.

It does it sometimes at idle when cold and does it just going down the road at 40-45mph when it gets into overdrive and is running in the 1500RPM range. It does not seem to do it when accelerating and I have not seen it do it at constant higher rpm's. Buy "seeing" it, I mean the tach needle jumps a little when it does it and I can hear it in the exhaust. So when it started, I figured it was due for a tune up and since then I have changed the distributor, cap, rotor, plugs, and wires and it did nothing! It still has the same problem and I cant figure it out. I checked timing and fuel pressure and everything seems good. I checked for codes and there are not any. Ideas?

 

[genynnc]

hmmm... no codes but it is OBD I. Which can be good and bad, depending.

How's the oil level? Notice any fuel smell from oil?

MPG, I assume it's not as good as could be/ used to be.

Have you tried cleaning the throttle body? (Actually removing it and cleaning it?)

The replacement distributor... was it just the ignition control module replaced or the entire unit?

Parts sourced from? (It's a [censored] shoot these days... even AC Delco stuff).

Do you have any exhaust issues? (My father has a '93 1500 350 TBI and it did something similar but it needed a new muffler and some other exhaust items. Replaced them and it cleaned up the idle and lower-rpm part throttle response/hesitation/miss.

 

[yonyon]

91 is a ways back, isn't that when they used to have that problem with cams wearing out all the time? A power balance test and a running compression test might suggest whether or not that's something to be concerned about.

Go after the simple stuff first. If you haven't replaced the spark plugs and cleaned the distributor cap in a while it's worth doing. Also check for intake gasket leaks with some carb spray. Those old paper gaskets don't last forever especially with TBI. Temporarily blocking off the EGR port with gasket paper can also rule out a dirty/worn EGR valve failing to seal.

When you changed out the cap and rotor did you check and see if there seemed to be a lot of play in the distributor shaft? If you're not sure what "a lot" is, just hook up a timing light as though you were going to set ignition base timing. If it jumps around all over the place your distributor is probably "dun wore out" so to speak and ready for a rebuild.

 

[morepwr]

Originally Posted By: genynnc
hmmm... no codes but it is OBD I. Which can be good and bad, depending.

How's the oil level? Notice any fuel smell from oil?

MPG, I assume it's not as good as could be/ used to be.

Have you tried cleaning the throttle body? (Actually removing it and cleaning it?)

The replacement distributor... was it just the ignition control module replaced or the entire unit?

Parts sourced from? (It's a [censored] shoot these days... even AC Delco stuff).

Do you have any exhaust issues? (My father has a '93 1500 350 TBI and it did something similar but it needed a new muffler and some other exhaust items. Replaced them and it cleaned up the idle and lower-rpm part throttle response/hesitation/miss.


The oil level is good and the oil is only about 1500 miles into its life. The distributor is a rebuilt unit from Autozone (complete)so I dont know where they source them but it runs exactly the same as with the old one.

The exhaust is a possibility and I will inspect that as best I can. I will make sure the throttle body is clean. It hasnt been super long since I last cleaned it though. And your right, the gas mileage isnt all that great anymore.

 

[Dakota1820]

are you sure it isnt a bad plug or wire?

 

[artificialist]

When I deal with injectors getting stuck open, 3/4 of them are on TBI engines.

 

[Jim Spahr]

Is it an auto trans? If so, I would look at the auto trans and especially the torque converter as a possible source causing the 'miss'.

 

[morepwr]

Well everyone, I looked over the truck again this evening and pulled out the plugs and looked them over. They look pretty good overall, there is some deposits that I know is some oil being burned.
The plugs are nowhere near being fouled and I double checked the gap and all is good there. The plug wires are new and all went on with a nice solid "click".

I guess maybe a compression check is in order to see if I have a cylinder that is fading away.

 

[wtd]

A bad fuel injector can cause a misfire. How many miles on the truck? You have replaced pretty much everything on the ignition side except the ignition controle module and the coil.

Put it on a scanner that can look at misfires and see if it is actually misfiring.

Wayne

 

[ms21043]

I've seen random misfire on GM HEI when the pickup coil in the distributor is on the way out...

 

[mattd]

Take a water bottle with a cap (poland springs etc), poke a hole in the top of the cap. Fill with water, spray on each header to see which does not "sizzle". When you have found one that doesnt, start your diagnosis there.

Or use a laser thermometer. I'd recommend a compression test. Usually with TBI injection, you wont have a misfire on only 1 cylinder.

 

[1kickbuttranger]

EGR getting stuck? I had a 91 Sierra and it had a mis right off idle. No codes. Replaced everything with no results. I ended up taking it to the dealer and it was a bad EGR casuing it to run lean.

 

[mattd]

After re-reading your OP, it does *sound* like it could be an EGR issue. Tap on the valve with a small ball peen hammer to see if the idle changes at all.