Really bad missfire!! I am stumped!

[cobbcfi]

Two Saturdays ago my truck started acting up (1999 5.7 250k) and I thought it could have been bad gas from the day before but two weeks later it's still struggling to get me to work and back.. I SeaFoamed it and that made it worse, changed the plugs and nothing. At idle, it does a little rocking around and that's it, put it under load and all he'll breaks loose!! Spitting, lunging, misfiring and shaking like hell. Let off the gas in neutral or park and it idles down OK but sitting in drive/reverse at a stop sign and it's bucking pretty good.. On the interstate it does OK sometimes trying to maintain speed but trying to speed up and it looses it's mind again.. Let off the gas and coast and I think it has stalled out, no shake and dead quiet. Then every once in a while for a second or two while struggling to get up to speed it takes off and runs perfect for the briefest of moments and then back to crap..

Any ideas would be greatly appreciated. .

 

[ezdaar]

Check timing.
Check fuel pressure, and or change fuel filter.


How many miles on engine?
Has it ever been rebuilt?
When you pulled old plugs what color was the insulators? Bone white! Brown, black?

 

[SunlitComet]

have you sought after any trouble codes active or stored?

 

[cobbcfi]

I had the codes checked and it came back as random miss-fire..

How do you check timing on my fuel injected truck?

It's has 260k miles.

My check engine light has on for some time like a year or so and the code was for an O2 sensor. Could a failed O2 cause this severe of a miis-fire?

 

[cobbcfi]

I have noticed the past couple of evenings when I am NOT running the A/C that the miss-fire is not as bad and periods of running OK last longer. During the day with A/C at full blast is when it runs the worst.

I know that the compressor puts a load on the engine but how could that cause it to run worse if it's a miss-fire?

 

[SunlitComet]

first off you can not adjust timing on the vortecs. you can only make sure the dizzy is in the right position that make the cam and crank sensors have a synced signal reference for the pcm. engine timing is controlled by the pcm and is out of your hands. if you do not have a p1345 coded floating around in there then the sensor sync is not really causing the issue. i would look over your spark plugs and inspect/test your wires/cap/rotor/coil for anomalies. start with that first.

 

[cobbcfi]

Replaced all of the plugs last weekend. I changed a plug and fired it up every time looking for the culprit cylinder and nothing changed with 8 new plugs..

The problem is that it doesn't make enough noise and miss-fire at idle to tell if one cylinder or another is the culprit. It idles OK, but as soon as you advance the throttle, it reminds you that something is dead wrong.

Anymore suggestions?

I may just take it to the shop tomorrow if they have time to look at it and let me know whats wrong..

Crank Position Sensor?
Faulty Wires?
Faulty Coil Pack?
Bad Fuel Pump? I have usually run at less then 1/2 tank for over 4 years.
Clogged Fuel Filter? Bad gas I got the day before?
Air Filter? I highly doubt this one.
MAF Sensor?

It should be (fuel delivery or ignition) otherwise the SeaFoam treatment wouldn't have made it worse if it was a blown gasket or timing issue..

This is gonna be a good one to figure out but it is still running, and having said that, I have now parked it until I can find the problem.

No reason to continue to force the truck to run and really damage it if it can be saved..

 

[SunlitComet]

if it was worse after seafoam have the shop check the map sensor data as well. it monitors the inside of the plenum. have them look at the misfire history as well. it keeps a running count of which cylinder is doing it and how often to help better isolate a problematic cylinder(s).

 

[God]

See what your cps looks like. It's located right under the harmonic balancer. Takes a minute to remove and replace

 

[Big Roller]

I'm gonna bump this thread -- My truck has started acting up like the OP's, but only does it on cold starts in the mornings. Has been doing it for the past two weeks. As soon as it's started, it sputters and spits... Until you put it in drive or reverse, then it jerks and bucks and goes crazy. It's giving me two codes, both of which are P1345. I just did a tune-up three months ago with ACDelco wires and plugs, cap and rotor. I put a new fuel filter on it a month prior to that. I called my mechanic and he said to check the plugs and see if the coolant level was full, so I pulled the plugs which looked good. I also tested the ignition coil and it is good too. It is also full on coolant. But, once it has been running for a while, the bad miss gradually gets better and better until it's gone. I don't know if it's just bad gas or if it's something else. I also checked the fuel pressure, which is 55psi at idle. I don't know where to start looking now! I read on another site that it could be the TPS sensor causing it? How do I check that?