Low RPM Stumble
Low RPM Stumble For the past few weeks, I've noticed what I'll call a "misfire" at idle on my 95 T/A LT1 (which seems to be pretty high-nominally +/- 1000 RPM) through roughly 2500 RPM. It is most noticeable at about 45 MPH or 2200 RPM. The car never wants to kill, but the stumble is awful under those conditions.
Last summer I replaced the stock Opti with an MSD unit because the water pump was shot and covered it in coolant. I also replaced the stock coil with an MSD coil. There is no coolant getting in the distributor or wires, and all wires test adequately and deliver spark to each cylinder as they should.
I have noticed some odd electrical discrepancies. For example, when the headlights are on at idle, voltage drops to around 12V at the needle (and at the alternator). Also, the RAP circuitry stopped working. Everything is grounded correctly at all points, and are clean and have a coating of dielectric to prevent shorts.
At this point I've pretty much narrowed the issue down to be either fuel or electrically related. I haven't checked the ICM or VR yet, but plan to tonight. Vacuum lines have already been checked and are fine. I will also check fuel pressure. Is there any other source for this issue that sounds familiar to anyone?
Last summer I replaced the stock Opti with an MSD unit because the water pump was shot and covered it in coolant. I also replaced the stock coil with an MSD coil. There is no coolant getting in the distributor or wires, and all wires test adequately and deliver spark to each cylinder as they should.
I have noticed some odd electrical discrepancies. For example, when the headlights are on at idle, voltage drops to around 12V at the needle (and at the alternator). Also, the RAP circuitry stopped working. Everything is grounded correctly at all points, and are clean and have a coating of dielectric to prevent shorts.
At this point I've pretty much narrowed the issue down to be either fuel or electrically related. I haven't checked the ICM or VR yet, but plan to tonight. Vacuum lines have already been checked and are fine. I will also check fuel pressure. Is there any other source for this issue that sounds familiar to anyone?
Guess I should also mention the car takes roughly 10 seconds to start when cold, which tells me it may also have a fuel pressure loss somewhere. This still doesn't account for the stumble unless I have a clogged injector(s) correct?
No tune on the car. It's completely stock except for the MSD optispark. The hesistation started after the car sat for about 2 months. I assume its a possibility an injector could have developed a clog while sitting and is now bleeding. I haven't been able to get the car apart enough to check the plugs for lack of time, but from what I can tell the misfire is on the right bank of the motor.
TECH Fanatic
Joined: Oct 2006
Posts: 1,106
Likes: 0
From: POULSBO WA.
My best way to find a missfire is to use a laser temp gun and check all the cylinders. U can find a missfire pretty easy. Do u know these cars are famous for a low rpm stumble from the factory, my car was tuned and given 8 more degress of timing...20 horse no stumble its great. But running on 7 cyl. Would also make a huge difference too.
Teching In
Joined: Nov 2011
Posts: 4
Likes: 0
I had a similar problem with my 95 LT1, and I never had an engine light. My car would bog down and stumble pretty bad around those speeds and RPMs you mentioned. I figured it was a misfire/improper ration of fuel and air. The real problem showed its face after a radiator replacement. The hose from the coolant inlet tube was bad at at the couple and cracked to the point where i could see it (and a small stream of coolant) after the cooling system pressurized. Constant flow of air into the cooling system caused my vehicle to choke on the air as well as caused some cooling issues. I swore it was my injectors, then my opti, or some sort of vacuum leak as well as a whole slew of possibilities that cause similar problems. drove me absolutely mad. Long story short, check the hoses carefully. You didn't mention temperature issues so im not sure if this will help but I wish you the best of luck.
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