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Old 02-29-2016, 11:35 AM   #1
duttch1
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99 Tahoe misfire...I know, I know a thousand threads...BUT!

I know there are 1000+ threads out there for this issue, I've read most if not all and I've done everything.....she still misses at idle, misses at high speed, misses going up a hill, hell it miss going 45 mph downtown! And it's always a random misfire.

As per all the forums and threads I've read, here's what I've done:

fuel pump (delco)
fuel filter
fuel pressure regulator (delco)
fuel pump relay (delco)
intake gaskets
fuel injection spider (delco)
crank position sensor
distributor complete (and delco cap&rotor)
plugs (delco)
wires (delco)
electronic ignition module (delco)
spark coil (delco)
downstream O2 sensors
upstream O2 sensors
catalytic converters

Basically everything I've read over the past 10 years that other people have done to correct this issue. My Tahoe remains stubborn and misfires.

This is of course a 1999 Chevrolet Tahoe, 2 door, LT, 5.7L, 4X4, 205,000 miles. It's been "coughing" since 132,000 miles.

PLEASE! Does someone have the answer???? I've done everything I've found online
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Old 02-29-2016, 01:45 PM   #2
j cAT
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Re: 99 Tahoe misfire...I know, I know a thousand threads...BUT!

first thing is the fuel pressure get a pressure gauge and make sure its 55-65PSI. 5.7L has higher pressures than the new 5.3L. also rev engine drive vehicle gauge connected see if it holds high. replace fuel filters @30-40Kmiles... next is the distributor remove cap and check for a rotor wobble. most fail at 175Kmiles. using a scan tool measure the long fuel trims should be less than 10%. also short term. if these read positive 15-25% it means a lean condition . then you check for intake manifold leaks . if no leaks and these still read high @2000 RPM then my guess would be the MAF is bad. MAF is not measuring the correct air flow into the engine. p0300 is what your saying ? injectors bad with a p0300 is not common . usually you get a p0306 code etc for a specific cylinder .also the fuel pressure will not hold key on eng. OFF..................
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Old 03-01-2016, 05:17 AM   #3
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Re: 99 Tahoe misfire...I know, I know a thousand threads...BUT!

Hey, thanks for the info j cAT. The engine light has never come on and it has never pulled a code when checked....I take that back, I did get a check engine light (service engine soon) and coded it and got an emission sensor fault. I was told it was probably just crappy gas. The engine light did not come back on the next morning and I have not seen it since. It has never come on due to the misfire, never given me a code.

The MAF sensor is one thing I have not replaced. I did a fuel pressure test about 7 years ago, then again about 2 years ago and it's always read dead on (according to the repair shop). I replaced the fuel pump anyway because my sending unit went out. That was 2 years ago and I pressure tested again after replacement pump was installed.

I replaced the distributer about 1 year ago and have replaced the cap and rotor at least 8 times each over the years. Strangely, a new cap and rotor always helped reduce the misfire (although not completely gone) for a month or two, then it always comes right back same as it was.

I did have the fuel injection spider replaced about 3 years ago even though there was no code. I was grasping at straws for "fixes" and that was next in line in the fuel system. At the same time the shop replaced all of the upper engine gaskets (manifold, intake, etc) because I was beginning to get the coolant drips down the block. The crap part of the $$ repair is every since then, at least once a week, it starts hard. It always started right up with just a bump of the key (despite the rough idle). But now, occasionally, the engine will crank for 2 or 3 seconds before firing up.

I was told it might be one of the new injectors leaking down the pressure after shutting the engine off....?? I have not verified yet.
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Old 03-01-2016, 07:48 AM   #4
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Re: 99 Tahoe misfire...I know, I know a thousand threads...BUT!

with a scan tool you have no miss fire history codes ? however you say you feel the vehicle missing when driving. could be some electrical problem could even be the ignition switch which is a somewhat common problem in these at this age and a few years ago. some have had ignition switch power problems to the tranny as well. replacing the cap rotor appears to fix it for a while could mean the cam gear is worn this can happen.most of these engines 175K miles the distributor needs replacing. GM made the distributor gear of a material that reduces the wear of the cam gear . if a replacement distributor has a harder material over time it may have worn the cam. this would throw off the timing . with a scan tool it is possible to observe the timing when driving see if its erratic. measure the fuel pressures/test procedure . injector leak down will show as hard warm start . ECT can cause poor cold starts . if ECT is OEM factory it should be replaced too old. if it is miss firing you will get CEL flashes . possible the PCM is crapping out .
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Old 03-01-2016, 08:49 AM   #5
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Re: 99 Tahoe misfire...I know, I know a thousand threads...BUT!

I've considered the ignition system and replaced several components but not the switch itself....that's an interesting thought. I'll do some investigating as per your recommendations, thank you!
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Old 03-01-2016, 11:13 AM   #6
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Re: 99 Tahoe misfire...I know, I know a thousand threads...BUT!

Quote:
Originally Posted by duttch1 View Post
I've considered the ignition system and replaced several components but not the switch itself....that's an interesting thought. I'll do some investigating as per your recommendations, thank you!
don't throw parts at it . some repair shops guess at your expense. OBD II with a good scan tool hooked up and driving around monitoring the component functions should get this narrowed down with little repair waste.
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Old 03-01-2016, 11:19 AM   #7
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Re: 99 Tahoe misfire...I know, I know a thousand threads...BUT!

Completely understood....and, I've been throwing parts at my Tahoe for years. I guess I should have been posting instead of just reading all these years. Thank you again
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