96 Chevrolet 1500 random misfire

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  #1  
Old 01-19-07, 06:13 AM
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96 Chevrolet 1500 random misfire

Hi All,

I usually visit the Electical Appliance section to help people there but I though I would post a question here about my pickup.

I have a 1996 Chevrolet C1500 with a 305 (5.0L) Vortech engine, 148K miles, 4L60E tranny. I am getting a random misfire running at steady speed or very slight deceleration ONLY. This has been happening for some time now and it is really starting to bother me now. If I let the engine misfire long enough, it will throw a P0300 code which is "Random Multiple Cylinder Misfire". It seems to be worse if the engine is running at higher RPM. If I run the pickup in second gear and hold steady about 2500RPM it is really bad. But, if I rev the engine with no load (parked) it does not misfire.

I have just installed the new MFI upgrade for other issues (poppet valves and fuel pressure regulator). I also just installed all new Delco plugs, wires, cap and rotor. I installed these about two weeks ago.

I thought about changing the coil next, but I will wait for some input before I tackle that. Otherwise, the pickup is running great! Acceleration is smooth and the idle is so smooth I can't even feel the engine running at stop lights.

Thanks in advance for any info.
 
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  #2  
Old 01-19-07, 06:32 AM
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does it miss on hard acceleration at any time?
have you retrieved any other codes other than a p0300?
 
  #3  
Old 01-19-07, 06:57 AM
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Thanks for the quick reply bejay. It does not miss at all during hard acceleration. It is as smooth as ever. She never misses at all during acceleration both light and hard. I could make the engine miss when driving steady speed and just even the slightest bit of acceleration, the engine smooths right out.

The very first time I started the truck after upgrading to the MFI, I got a code pointing to the MAP sensor. I never got that code before and It has not come back since. So I think it was just a fluke. I am getting no other codes.

This misfire condition was happening long before I changed out the parts. I am on my second set of plugs, wires, cap and rotor since I have been having this problem. When I changed them the first time, it seemed like the misfiring went away but I don't think it really did. That was so long ago now it is hard to remember.

Also, when I drive the truck at highway speed (65-70 mph) it does not seem like there is a misfire problem. It is only at slower speeds below 45-50 mph it seems like it is a problem. Maybe it is due to the increased load on the engine at highway speeds, who knows........

Thanks again.
 
  #4  
Old 01-19-07, 06:36 PM
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hmm, sounds like what my 86 pulsar used to do, all the same symptoms. so i changed the coil. no apparent improvement at first, but after driving around for about 15 - 20 minutes, it went away. sure enough i put the old coil back on and the problem came back in 15-20 minutes.

keep in mind this car was carbureted, not fuel injected.
 
  #5  
Old 01-19-07, 06:43 PM
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dont see the coil as causing the problem as it would likely miss under heavy load or could even cause cold start up problems,
might check the distributer shaft for excessive play and would look at the egr maybe drive it for awhile with the egr disconnected to see if it still misses it will cause a check engine light to come on while you are driving it with it disconnected if it doesnt miss with the egr disconnected it doesnt neccasarly mean the egr is bad but possible, it could also be computer programming issues causing the egr to be opened up to much at the wrong rpms.
 
  #6  
Old 01-25-07, 07:12 PM
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I have replaced several fuel pumps on these trucks to correct this condition.
First change the filter.The only test is to put it on a dyno, or what i did is put an extra long hose on my fuel pressure gage and run it into the cab and drive it down the road. when you put it under load, the tired pump can't keep up. If it is a pump, it is mounted in the tank and the tank has to be dropped, or what I do is unbolt and jack up the box. It is much easier.
 
  #7  
Old 01-26-07, 05:29 AM
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thanks for the reply themechanic43. Would a fuel pump cause the engine to miss only running at steady speed though? If I accelerate both slow and hard, it is smooth as it can be. It does not miss at all during acceleration.

The next thing I am going to try is checking the fuel pressure. The truck does have 148K miles so anything is possible. The filter was changed recently. I have been having this issue for about the last 2-3 years and I have changed the fuel filter several times since then. I think it got worse since they switched to the Ethanol blend fuel here.

Thanks again
 
  #8  
Old 01-26-07, 07:32 AM
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miss

have you tried cleaning the injectors and the intake
 
  #9  
Old 01-26-07, 05:31 PM
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I have also experinced this problem start with the fuel filter it is cheap and easy to do also you said this only happens around 45-50 mph this is the speed range that the tourque converter goes into lock up mode watch the tach when this happens it may be just a converter lock up thing that is being felt try driving this in 3rd gear and see if the problem happens then dont know that there is anything that could be done for that and does it a missfire code all the time. Also and not too many people know this but there is some slight adjustments that can be made to the timing but it requires a scantool too do it that will read engine timing that may be something that a reputable shop could help you with unless you have access to the tools not something you can do yourself
 
  #10  
Old 01-26-07, 06:13 PM
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thanks for the replies.

I upgraded the injection system to the new MFI a few weeks ago. So the injection system is new and I have run seafoam through the engine before I changed out the plugs, wires, cap and rotor.

The fuel filter was changed at the last oil change. It has been about 3000 miles now.

I have been having this problem for about 3 years or so. It seems like it is getting worse now.

It misses worse if I run in 2nd gear and hold the engine at about 3000-3500 rpm. It seems like the engine only misses at a certian load, no matter what gear or speed I am running.

If I run 70-75 mph on flat road, it doesn't miss. If I put the cruise control on at this speed, I can feel it missing as I go down a slight hill such as the down side of an over pass.

It could be a timing problem. I guess I may have to just give this one up and take it in to have it looked at.
 
  #11  
Old 02-02-07, 05:10 AM
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Timing

Well, I finally gave up and took it to a shop. About 4 years ago I had the intake re-sealed because it was leaking where they normally leak from and the mechanic didn't get the timing set right on the distributor. It was supposed to be at 0 degrees and he said the timing was retarded 10 degrees. He adjusted it and drove the pickup a couple of times and it seems like the problem went away. He will drive it again this morning to double check but it looks like that was the problem.

Thanks for all the input. I will post back with the final results.

It looks like that was the problem. I put enough miles on my pickup today to determine if the problem still existed. I didn't notice a single misfire
 

Last edited by gdoug; 02-03-07 at 08:09 PM.
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