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intermittent misfire


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#1 Kart29

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Posted 03 January 2007 - 10:49 AM

I've been having a problem with my `99 Silverado 5.3L. I've had it to the mechanic several times but nothing has been done to fix the problem.

Description of problem:

There is an intermittent misfire that makes the engine run very rough. When running smooth, the engine runs great. The misfire comes and goes at random. It is like turning a switch on and off, or flipping the switch between rough and smooth. The misfire may occur for just a split second or two or it may last for several minutes. The misfire occurs at random when idling, accelerating, cruising, after decelerating.

My mechanic has read codes from history that show P0300 - random misfire. P0303 - cyl. 3 misfire, P0305 - cyl. 5 misfire. Last night I took the truck to the auto parts store and they read the codes P0137 - post-cat O2 sensor low voltage (lean?) and P0305 - cyl 5 misfire.

Corrective action to date:

So far, the mechanic has cleaned all injectors and replaced the #5 injector. Problem remains the same. Although the problem occurs every time I drive the car, my mechanic can't make the misfire occur so he can identify the problem - it always runs great when he drives it or has it on the computer.

Last night I replaced PCV valve, cleaned MAF sensor, replaced fuel filter. No change in problem.

Any ideas? I'm especially confused by the O2 sensor code. I don't know if the misfire could cause the P0137 code or if a bad O2 sensor could cause the misfire. Also, I didn't think the post-cat O2 sensor was used for engine management but only to identify a problem with the catalytic convertor.

I'm thinking maybe I'll just try replacing the coil for cyl. #5 and see if that fixes it. Or do you think maybe I should try replacing the driver's side O2 sensors first?

#2 Kart29

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Posted 03 January 2007 - 11:17 AM

Oh yeah, I replaced all plugs and wires, too.

#3 99Silver6.0

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Posted 03 January 2007 - 05:42 PM

You may need a Crank Relearn done. Bad CPS signal can make your truck run like dookie. This usually results from the timing chain gaining a bit of slack in it over time. A GM stealership can do the relearn with a TechII. Or if someone you know has HPTuners or EFILive they can do it.

How many miles your truck have?
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#4 95Sierra2500

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Posted 03 January 2007 - 07:30 PM

You don't mention how many miles are on your truck, but perhaps you've got a bad cat. I doubt it's a bad O2 sensor causing your problems, but a partially clogged cat could do it. The Crank Relearn procedure would also be a good place to look at.
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#5 Mr. Smith

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Posted 04 January 2007 - 12:11 AM

Instead of changing parts and spending money why not swap coils to another cylinder to see if you get a code for another cylinder? They are all the same. And if you get another misfire code for that cylinder you swapped the coil(s) to you probably found a problem

#6 Kart29

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Posted 04 January 2007 - 09:02 AM

I have 155,000 miles on my truck.

If slack in the timing chain caused a problem with the crank position sensor, wouldn't the truck run poorly all the time?

This problem is an on/off type situation. When it's running smooth, it idles smooth as silk with not even a hiccup. The accelartion is crisp with good throttle response. But when it runs rough, it runs terrible. It will switch instantly between rough and smooth running. This makes me think it's electrical in nature more than mechanical.

If there was a general, constant condition (such as a failing catalytic convertor or timing problem) it seems the problem would be constant or at least run on a spectrum from great, to good, to fair, to bad, to lousy. But that's not what it's doing. It's only either perfect or lousy and will switch between the two in a split second.

My mechanic has tried moving coils around but the misfire codes don't always isolate a particular cylinder anyway so I'm not sure that tells us anything. One time, the code showed radom misfire and cylinder 5 misfire. Another time it showed cylinder 3 and cylinder 5 misfires. Another time it showed cylinder 5 misfire and 02 sensor low voltage. The only consistancy seems to be that the codes are always report a problem on the DS cyl. bank.

Do you still think it is likely to be a problem with the CPS or cat?

#7 99Silver6.0

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Posted 04 January 2007 - 06:16 PM

I have 155,000 miles on my truck.

If slack in the timing chain caused a problem with the crank position sensor, wouldn't the truck run poorly all the time? 

This problem is an on/off type situation.  When it's running smooth, it idles smooth as silk with not even a hiccup.  The accelartion is crisp with good throttle response.  But when it runs rough, it runs terrible.  It will switch instantly between rough and smooth running.  This makes me think it's electrical in nature more than mechanical.

If there was a general, constant condition (such as a failing catalytic convertor or timing problem) it seems the problem would be constant or at least  run on a spectrum from great, to good, to fair, to bad, to lousy.  But that's not what it's doing.  It's only either perfect or lousy and will switch between the two in a split second.

My mechanic has tried moving coils around but the misfire codes don't always isolate a particular cylinder anyway so I'm not sure that tells us anything.  One time, the code showed radom misfire and cylinder 5 misfire.  Another time it showed cylinder 3 and cylinder 5 misfires.  Another time it showed cylinder 5 misfire and 02 sensor low voltage.  The only consistancy seems to be that the codes are always report a problem on the DS cyl. bank.

Do you still think it is likely to be a problem with the CPS or cat?

<{POST_SNAPBACK}>


It can be a sporadic thing. My old 5.3 did the same thing. I ended up pulling the motor and dropping a 6.0 before I had a chance to play with it much. But the chain on my 5.3 was pretty loose!!!

You by chance try moving around injectors?????????? If you have 1 or 2 bad ones.... I have 7 of my old 1999 19# injectors!!

Edited by 99Silver6.0, 04 January 2007 - 06:17 PM.

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#8 Kart29

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Posted 05 January 2007 - 08:43 AM

Hmmm. I may have to check on how much the crank relearn procedure costs.

Last night I got the code read again at the auto parts store. This time only one code -P0305 - cyl. 5 misfire again. So, I came home and swapped coils between cyl. 5 and cyl. 4. If it's a coil I would think this would identify the problem. I also made the swap on opposing cylinder banks this time rather than just on adjacent cylinders. If the error codes move to the other cylinder bank I should be able to eliminate O2 sensors and cat convertors.

I've already replaced the injector on cyl. 5 so that shouldn't be the problem. If I do narrow it down to an injector problem I'll be in contact, Jared. Thanks.

I pulled the neg. batt. cable to clear the code.

A test drive last night and driving to work this morning has produced nary a hiccup. The problem went away for about a month after the first time I took it to the mechanic. Maybe the problem has gone into remission again. We'll see.

#9 DEZYNER

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Posted 10 January 2007 - 03:08 PM

We had a similar problem that we just could not track down, then the check engine light came on flashing which is supposed to indicate a mechanical failure, injectors, wires etc.

Turns out the intake manifold bolts had come loose and the gasket had a vacuum leak. This is a 03 Yukon with 61K. Well $600 later the dealer changed the gasket and the problem seems to be fixed. FYI - intake gaskets aren't covered by warranty, according to GMC its a normal "wear" item.

What a bunch of crap. I'm contacting GMC about a partial reimbursement.

Anyone else have their intake come loose and start a vacuum leak? My dealer seemed to think its a pretty common problem. I challanged his diagnoses, as my caprice survived 17 years with no such problem.

#10 red00sierra

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Posted 14 January 2007 - 10:12 AM

Hmmm.  I may have to check on how much the crank relearn procedure costs.

Last night I got the code read again at the auto parts store.  This time only one code -P0305 - cyl. 5 misfire again.  So, I came home and swapped coils between cyl. 5 and cyl. 4.  If it's a coil I would think this would identify the problem.  I also made the swap on opposing cylinder banks this time rather than just on adjacent cylinders.  If the error codes move to the other cylinder bank I should be able to eliminate O2 sensors and cat convertors.

I've already replaced the injector on cyl. 5 so that shouldn't be the problem.  If I do narrow it down to an injector problem I'll be in contact, Jared.  Thanks.

I pulled the neg. batt. cable to clear the code.

A test drive last night and driving to work this morning has produced nary a hiccup.  The problem went away for about a month after the first time I took it to the mechanic.  Maybe the problem has gone into remission again.  We'll see.

<{POST_SNAPBACK}>


kart29 did you find out what the problem is? i am having the same problem with my 2000 sierra 1500 with the 4.8.

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#11 rev01

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Posted 14 January 2007 - 04:46 PM

hey i'm having a simular problem with my 99 silverado except now it runs rough all the time now and even goes to a high idle and then dies by its self anyone have a clue

#12 Black Bart

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Posted 18 January 2007 - 12:20 PM

You will probably find that it is indeed a intake manifold
gasket problem. A freind of mine had a 00 tahoe that
gave the same codes and after changing a 02 sensor or
two with no success we changed the intake gasket and
that cured the problem. About two hours time and $85
for the gasket.

#13 Samw

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Posted 17 August 2009 - 09:08 PM

2002 GMC Yukon Denali XL 116000 miles
I am also having a similar problem. I am getting two codes, p0137 o2 sensor bank 1 sensor 2. P0308 mist fire cylinder 8 detected. This misfire only seems to happen when I am pulling my boat, if I am going up a hill and it drops into passing gear in cruise RPM's hit 4500 it flashes the code. I have also had a map code. At idle (500 rpm's runs real ruff. Take it off idle it seems to run fine. I was thinking it was the o2 sensor but now not sure.
How do you test for a intake leak?
Thanks for any help




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