View Full Version : Hesitation/misfire


ZexGX
09-16-2008, 06:27 PM
Getting a random misfire... On startup it sometimes revs high, then idle goes down and it lopes (stock GM cam), then stalls unless I give it some gas. When driving along at speed, it'll miss ocasionally and even more under load. When this happens (and when it lopes), RPMs/volts will drop in sync. Volts are fine when running normally before this happens.

I have changed:
MAF (bad - replaced with new which helped and stopped the error codes - also tested the new one by unplugging)
Fuel filter (was good - keeping the old as a spare)
Front O2s
Plugs + wires (I can get pictures if anybody wants of the removed parts)
Air filter is K&N drop-in and has been recharged recently
Two coils have been replaced and the control module have been replaced twice recently!
Camshaft position sensor has been recently replaced with proper matching part

I am thinking it's either the fuel injectors or the TPS. I'm open to other ideas. If you think it's fuel injectors or the throttle position sensor (?), how can I check or have it checked? I have been trying to fix this for weeks now and it's driving me crazy! :bang: The MAF screen may be SLIGHTLY bent (no fault of my own - have pics if requested) but it is clean and air flows through. The MAF sensor on the V6's are removable units housed inside the throttlebody before the valve. Not sure if the bent screen would cause this. It was running fine for a LONG time with the bent screen.

Car is a '99 with 115k miles on it. It has an electronic throttlebody (drive by wire).

It's a 3.8L V6 f-body but I figure maintenance is maintenance and an F-body is an F-body. I've asked in the V6 forums but haven't gotten much feedback.

P.S. NO CODES HAVE BEEN THROWN :bang:

LT1Formula26
09-16-2008, 11:14 PM
i am having a similar problem, but mine is when I push the pedal all the way down really quick, it hesitates or has even popped. Really weird.

ZexGX
09-18-2008, 11:31 AM
No ideas?? This car is my DD right now so having it not running properly is a pain... At this point I'm thinking fuel pump or injectors, cause it can stall immediately after starting up, as if fuel was cut to it. Also when I was on a roadtrip recently, I'd come to a stop but still hear a loud whirring sound coming from the rear end that would eventually go away. Oh yeah, and the fuel gauge is sometimes wacky (shows more than there is, or less than there is), and for some reason when I put gas in it sometimes (at 1/4 or less), the pressure stop on the pump activates after like 0.4 GALLONS...

Is there a way to test the fuel pump/fuel injectors?

Scoggin Dickey
09-18-2008, 11:55 AM
Sounds like a vacuum leak or something similar. Where do you get that it's misfiring if no codes have showed up?

nemss1
09-18-2008, 12:19 PM
there are current threads on misfire in the other secitons/forums

ZexGX
09-18-2008, 04:59 PM
Sounds like a vacuum leak or something similar. Where do you get that it's misfiring if no codes have showed up?

Well, it may not be caused by a misfire. It's sorta like a "popping" motion, and its chances of occuring increase with the throttle applied. Feels like it just loses power (like throttle dropping to 0%) and then instantly regains it (like a switch gets flipped) but it happens in a fraction of a second - faster than I can duplicate it by letting go of the gas and then punching it. I suppose it could be the fuel pump or an injector(s) now and its just starving for fuel. I checked all of the visible vacuum lines and they looked fine and were connected. I suppose I can check the other engine-specific forums for troubleshooting, but I figured general maintenance/troubleshooting would be able to provide some ideas for a resolution/cause to the problem.

peterock98
09-21-2008, 09:06 PM
Bad spark wire or wires.I see you wrote you replaced them what kind of wire did you go with.

ZexGX
09-22-2008, 01:24 AM
Brand new Bosch pre-cut lengths & AC Delco Rapidfire plugs.

peterock98
09-22-2008, 07:53 AM
I had the same problem I brought some cheap wires and my car had a bad misfire.My check engine light was flashing when I drove.I installed the msd's and so far so good.

ZexGX
09-22-2008, 10:50 AM
Yep... No check engine light and it's not 100% of the time... Only SOMETIMES on startup, idle, and increasing load - I also noticed last night that it increases when going uphill... Gonna throw on a fuel pressure gauge soon.

peterock98
09-22-2008, 04:43 PM
Might be all of the crap there putting in the gas today...

Barr6505
09-24-2008, 12:21 PM
I am currently trying to fix a similar problem on my 99 formula with 101,000 miles. It will hesitate under load and lose all of the power. I thought fuel pump and just checked the pressure. 57lbs so it isn't that. I was thinking injectors. I recently did a tune up to try and fix the problem and I had three plugs with black residue on them. Not oil residue, just sooty. Mine was doing this before I put the plugs and wires on so I don't think the cheap wires and plugs messed it up. :)

Anyone replaced a fuel pressure regulator on an LS1 with similar miles? Ignition control module?

*Oh, and all of my o2 sensors are throwing codes. I believe they were throwing rich and lean, but I can't remember what the guy at Autozone said.

ghardester
09-24-2008, 10:31 PM
Bad spark plug wires. Or you have another bad coil. Electrical problems tend to show up more under a load, when you are trying to excelerate. Go back and be sure of the plugs, coil, wires.