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Misfire cylinder five with load on engine

Tiny
HOUSEOFBRUGHS
  • MEMBER
  • 1999 CHEVROLET TAHOE
I have the SUV listed above LS 4d 4wd with about 86,000 miles 5.7 V8.

Since I purchased this truck with 52,000 miles on it I have been having this same problem repeatedly. The dealership where I purchased it "tried" to fix it with the diagnoses of misfire on cylinder 5 several times with no luck and recently I took it to a GM dealership and they said it would just be a shot in the dark until we find the part causing it. Since the diagnostic machine does not give a definitive reason this is happening. I don't have 1000's of dollars to keep replacing parts until we hit the right one so I was hoping you may have some better insight than what I have been getting.

Here is what is happening: When I accelerate it does just fine when I reach between 1,300 to 1,500 RPM's and 55-60 MPH the engine shakes like the motor mounts are broken. If you press the gas and change the RPM's or move past 60 MPH via passing gear not steady RPM gain it goes away gradually. This happens on the slightest of hills now if I am climbing a larger hill or towing. Basically any kind of load on the motor it is horrible. I have changed plugs, wires, fuel filter, DC and RB. I hope someone has some good information on what I can do to fix this. I want to keep the truck so come on guys help me out!

Michelle
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Sunday, October 28th, 2007 AT 8:30 PM

17 Replies

Tiny
BLACKOP555
  • MECHANIC
  • 10,371 POSTS
One of the easiest things to check is to make sure the fuel injector is not plugged, also check the distributor cap and rotor for cracks melted point on cylinder 5 or excessive carbon build up.

Also test for compression at that cylinder to see if its less then the rest. Also check for spark at that cylinder.
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Sunday, October 28th, 2007 AT 8:44 PM
Tiny
HOUSEOFBRUGHS
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The distributor Cap and Rotor button are new. I have not checked compression or spark to that cylinder yet. If either of those were bad would I only be having this problem with a load on the motor at those specific RPM's as mentioned in my orginial post?
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Sunday, October 28th, 2007 AT 8:50 PM
Tiny
BLACKOP555
  • MECHANIC
  • 10,371 POSTS
That could be very likely.
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Sunday, October 28th, 2007 AT 9:47 PM
Tiny
2CARPRO JACK
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99 Tahoes had CPI engines. You need an injector. Gm kept kinda quiet on the fact that they were warrantying them to 100k. Big problem with that engine. Also had problems with the fuel pressure regulators.
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Monday, October 29th, 2007 AT 7:43 AM
Tiny
HOUSEOFBRUGHS
  • MEMBER
  • 5 POSTS
Thanks Jack! So I would still be under warranty then since I have not breached the 100K mark yet or did they have a certain amount of years that went along with that like 100k or 5 years which ever comes first. Etc? I guess the next thing will be finding a dealership that will do the warranty work. My local Chevy dealership didn't even want to look at it.

Probably wouldn't hurt to have the FPR checked out as well I would imagine.
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Monday, October 29th, 2007 AT 8:37 AM
Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
If you are a squeeky enough wheel, you might get greased. This far along it is kind of at the dealers discretion
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Monday, October 29th, 2007 AT 9:36 AM
Tiny
HOUSEOFBRUGHS
  • MEMBER
  • 5 POSTS
Thank You again for your help. You have been wonderful! I spent 45 mins on the phone with Chevy. I have that squeaky wheel rolling now. They have contacted the dealership and guess what they are suddenly willing to look at it. At least that is a step forward with Chevy involved. I will let you know the outcome.
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Monday, October 29th, 2007 AT 10:04 AM
Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
Go get'm
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Monday, October 29th, 2007 AT 10:20 AM
Tiny
999TAHOE
  • MEMBER
  • 5 POSTS
My '99 Tahoe does basically what you were describing. Did that remedy your problem?
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Sunday, December 16th, 2007 AT 3:58 PM
Tiny
999TAHOE
  • MEMBER
  • 5 POSTS
155,000 miles, random misfires accelerating through 68-72mph to pass, on hill, with trailor.
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Sunday, December 16th, 2007 AT 4:00 PM
Tiny
HOUSEOFBRUGHS
  • MEMBER
  • 5 POSTS
Here is the update.I took it to my local GM dealership. They kept it for 3 days and after they exhausted all of their "guesses" they contacted the GM engineers who continued to scratch their heads. I was given these options from GM you could start with replacing all of the injectors and a valve job (but we can't gurantee that will fix it) or you could replace the motor (we also can't gurantee that will fix it either) we can give you a 500.00 brand loyalty certificate and you can go buy a new one. That will fix it. (There lame attempt to make this a laughing matter) I told them that the brand loyalty offer of 500.00 was offensive to my sensibilties! I have dumped well more than that into this POS to try to find someone who knew what was going on. I guess when it boils down to it if the GM Techs & Engineers have no clue. No body else will either unless they just get lucky. I personally think GM knows this was an across the board problem that they just don't want to deal with.

Bottom line.I traded in the next day on a Chrysler!

My advice to anyone else. Good luck on finding the problem. Smartest thing to do is dump it and get something else if you can.
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Sunday, December 16th, 2007 AT 7:23 PM
Tiny
999TAHOE
  • MEMBER
  • 5 POSTS
My mechanic who I consider intelligent and trust said he has done some research and said it may be carbon built up on the valve guide that, when under a good load and therefore hot, may expand and touch or obstruct a valve and make the missfire occur. I'm not a gear head and may not have said all of that correct.

It may also have to do with the time I was at the usuall oil change place and got tired of the guys trying to sell me crap so I bought into a sludge flush out treatment hoping they would leave me alone in the future. I'm sure all it did was just stirr around all sorts of good stuff and subsequently damaged the engine.

I do know it's not the gas (current tank), fuel pump, fuel filter, mass airflow sensor, coil, wires or plugs. Haven't gotten around to the injector yet; maybe all that cheap wal-mart gas has caught up.
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Sunday, December 16th, 2007 AT 7:51 PM
Tiny
999TAHOE
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Forgot one thing.I also had a head gasket recetly replaced. I've read on some forums that they thought it was water in the oil causing the problem. No dice
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Sunday, December 16th, 2007 AT 7:54 PM
Tiny
2CARPRO JACK
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You never mentioned the head gasket before. I have done so many of those I dont care to recall them! But when the top of those are taken apart the outer valve on the injectors sticks and causes misses almost every time. The updated injector only has one valve on it instead of two.
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Monday, December 17th, 2007 AT 7:17 AM
Tiny
PEAR69
  • MECHANIC
  • 1,482 POSTS
999tahoe, My engine did exactly the same thing. It turned out to be a sticking exaust valve just as you described. I just replaced the heads with GM performance vortec heads. These heads come complete and assembled. You can just bolt them on. However I matched ported and smooth polished the ports myself and got significant performance gains. These heads cost about $570.00 for two and I got them from Jegs performance parts. On-line
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Monday, December 31st, 2007 AT 12:08 PM
Tiny
CHARLES COLLINS2
  • MEMBER
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My 1999 Tahoe 4x4 sport utility got a P0203 code. The P0203 is a diagnostic trouble code that is set when the engine control module (ECM) has detected a fault in the injector circuit for cylinder 3. IT also got the P0305 code means OBD-II code that the engine control module (ECM) detected a misfire on cylinder number 5 of the engine.
Notice it all circles around the ECM.
My guess is if it is older than 10 years old, it is way past time to replace the ECM and injectors.
Injectors can go bad anytime but I have 234,000 miles and mine work fine.
The code was reset and the issue went away.
I am going to replace MAF (Mass Air Flow) sensor and my four O2 sensors.
We did that on my Sons 2002 which was acting as you described and it fixed the issue.
360,000 miles later and it still runs perfect.
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Wednesday, August 19th, 2020 AT 5:04 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Just to be clear, the P0203 and P0305 are set at the same time and does the engine run rough? I am just trying to understand what you are currently dealing with.

The idea that it is time to replace the ECM is not sound. The ECM doesn't wear out and require replacement at a certain time. It very well could be the issue on this but I suspect we have two separate issues on these cylinders.

It could be related but we need to address one of them first and see if it corrects the other. I would suggest tackling the misfire code on cylinder 5 and seeing if that corrects the other.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Let us know what you find and we can go from there. Thanks
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Wednesday, August 19th, 2020 AT 6:27 PM

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