robertm

Having an intermittant engine stumble/stall problem on my LS1. It's seeming to happen when the system is cold.

It happens when I put it in gear and release the clutch. When disengaged, I can rev the engine and it runs perfectly. However, the second I release just a BIT of clutch, it stumbles and just about dies. Feel can be described as bucking, like when you first learned to drive standard and had jerky clutch release. It also will appear at times while moving if I step on it while it's in its bad mood. In that case, it feels more of a miss than anything else.

This was all that happened for the first day of it's problem. Now it's added the feature where the tach goes to 0 while this happens. All the other gauges work, and the engine is running. While the tach is at 0, I cannot do anything. Attempt to make forward progress result in the car bucking. But when the tach comes back, it runs fine.

The mod to the car would be headers. But they were installed way back in March, and the problem just appeared Wednesday. I believe the guy who installed it tuned out the rear O2 sensors(along w/ the skip shift)

We've checked for and fixed one vacuum leak. Didn't help. There is an exhaust leak well after the O2 sensor and cat on the passenger side, right in front of the Y

I suspected possible fuel pump failure or bad gas, but neither of those would cause the tach to zero. There are no codes set. I'm about to purchase a scanner for my laptop to grab data while the thing bucks. It only appears while in motion. I'm assuming that it is a factor of the load more than the motion. Going to attempt to engage the parking brake and load the engine next time it does this to further diagnose.

Any tips or advice?
Thanks!

It smells like the PCM is acting out. But I'm hoping perhaps there is a simpler cheaper thing to try first.

robertm

After more searching, I found the "check the wires" post. The rear drivers side was loose. This will be great if that's the problem! Does that explain the tach zeroing out though?

Segond56

I would def. lean in the direction of a bad ground or exposed wire, probably a starter or alternator wire. I had a similar problem on a different car where it would cut out from time to time but only while driving and it turned out to be the starter wire where the coating rubbed through and was hitting on the block. This only happened when there was some forces on the motor to get it to move the little bit and contact the exposed area. I would look over all those wires first. Hope this helps.

robertm

Yeah. The loose plug wire was NOT it.

You mentioned the starter wire. That is currently in contact w/ the header. I tried to get it out of the way, but it's too short. The wire is in the plastic inside the heat tape, but I bet it melted inside. I dismissed it as a problem because it starts just fine. I'll take a further look.

HAZ-Matt

iTrader: (8)

I have an automatic so this may or may not be helpful.

The only time I had it act like it would buck and the tach went to zero was when the connector to the crankshaft position sensor had been damaged. It was an intermittent problem. I ended up replacing the connector (soldering the new one to the existing wiring harness).

I think the camshaft position sensor is supposedly used for input for the tach as well, but the wiring to the crank position sensor is more commonly damaged. On mine it was after I had the motor mounts replaced.

tyler83

I am having this exact same problems on my 02 z28 m6 with lt headers. All the symptoms you describe is right on with the ones i'm having. I'm very interested to see what you figure out on this issue. I am also right at 100000 miles with original plug wires so dissappointed that didnt fix it, i have noticed when the car is in a "bad mood" that it sounds and feels like its only running on 7 cylinders.

robertm

Edit: Just realized this should have been in external engine section. Sorry moderators, won't mind if it moves.

I read that the crankshaft position sensor is near the starter. Which means it might share the plastic run up the passenger side of the engine, which is touching the headers. Will jack up the car tomorrow, detach the wires(hope to at least) so I can pull the wire up and investigate and tape. I'll take pictures to post if it turns out to be true.

I was told there is some sort of insulating exhaust tape you can place on the headers to protect the wires that have the misfortune of coming into contact due to their short length. Anyone else use that or have experience with lengthing the harness?

Thanks

robertm

Found the most likely problem. Haven't fixed it just yet, but its painfully obvious. Check the harness coming from the PCM that heads down to the starter area. It contains the oil sensor, crank position sensor, O2 sensor, and ground. I verified that 2 of the CPS sensor wires are bare, and some of the O2 as well. The ground is in tact though.

To get this harness out, you have to remove the starter. The CPS sensor plugs in above it. There is also what seems like a ground attached to the starter as well.

trans665

good find, im going to ck on that stuff right now also.

LS6427

iTrader: (2)

Exactly why I'm buying a 100% complete all new wiring harness for when my new engine goes. Coming up on 14 years old.......I'm bound to have this type of issue ion the coming years if I keep the factory harness.

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robertm

That fixed it. I had to mount the starter 3 times. Once put the harness on the wrong side of the header, would have melted a new part. Once had the ground(or whatever that wire to the starter is) touching the header. Third time was the charm.

Started right away, and was smooth like it normally is. Problem is to figure out if this is your problem, you have to remove your starter. Fun.

rabbit320

iTrader: (11)

if that doesnt fix the tach, check fuses, especially the ones inside the car driver side of dash