Engine Misfire after Intake Manifold Replacement

[chrisvt]

2000 4.3l V6 Silverado. 112k miles. Regular cab, 8 foot bed.

 

A few months ago I had the intake manifold replaced on my truck. On the first drive after getting it back from the mechanic, I noticed some engine skipping/stuttering/misfiring between 40-50 mph. The stuttering was very quick (less than a second) but the RPMs would drop quickly and you could feel the truck buck. This would only occur on the ride to work in the morning and would not happen on the ride home.

 

I brought the truck back to the mechanic who looked the vehicle over. There were no misfire codes on the computer and the O2 sensors were fine. He figured that because it only happened in the morning when everything was damp, that it might be electrical. We did a full tune up (new plugs and wires, coils, distributor cap and rotor) and the stuttering stopped. This coincided with warmer. dry weather.

 

Everything was fine for about 3 weeks until we received 1.5 inches of rain in one day. The very next morning the stuttering returned and is still occurring (it's been wet weather ever since).

 

This wasn't a problem until the intake manifold was replaced and seems to surface with wet weather. I've googled and it could be anything from a torque converter lock up issue to a fuel pump, although none of those causes make any sense given that I had 3 weeks of problem free driving after the tune up.

 

Anything ideas what could be causing this?

 

 

[rjgti]

lean condition from intake leak

[chrisvt]

Thanks for the reply. Is there someplace I should be looking for loose hoses or will it require the disassembly of the manifold?

[rjgti]

I usually will use propane to look for vac leaks and listen for a pickup in idle

[chrisvt]

I figured I should follow-up given that I think I resolved this issue.

 

A week or so after my initial post, my girlfriend was driving behind me and noticed that whenever the truck bucked, black smoke would come out of the tailpipe. We also noticed that the exhaust would occasionally shoot black smoke even when there was no bucking and the the truck felt as if it were running smoothly.

 

Figuring that this was an issue with un-burned gas, I stopped by the dealership and had them run a few diagnostic tests. Everything checked out okay except for the mass air flow sensor. They wanted $400+ to fix it, which I declined. I ordered a new sensor off ebay for $32 and had it installed in 5 minutes. Since then, the bucking has stopped as has the black smoke from the exhaust.