Running Rough when hot, stumbling, missing.....
Thread Starter
3rd Gear
Joined: Jul 2009
Posts: 3
Likes: 0
From: Clayton CA
Running Rough when hot, stumbling, missing.....
Hi All,
Well after buying my '94 Corvette a few weeks ago, I noticed on a really hot day (95+ degrees), that it would start to run really rough and stumble when the temperature got up to around 3/4 on the analog gauge. I had my mechanic check the fans and cooling system and he noticed no issues. He said the fans were turning on at the proper temperatures, etc.
This morning I replaced all the plugs/wires to try and fix the problem. After about a 6 hour job, it was all done, and ran excellent, much better than before, smoother and no stumbling. Later today I took the Vette out for an errand (another 95+ degree day). After sitting at a stoplight for quite some time, the temperature got up past 3/4 and once hit 260. I took off from the light and it started stumbling really bad, worse than it has ever been. No power, running really rough through all the gears.
The other things I've been told that it could possibly be is the dreaded OptiSpark or possibly the water pump.
I'm trying to determine what I should do next. Your opinions/expertise is greatly appreciated!
Well after buying my '94 Corvette a few weeks ago, I noticed on a really hot day (95+ degrees), that it would start to run really rough and stumble when the temperature got up to around 3/4 on the analog gauge. I had my mechanic check the fans and cooling system and he noticed no issues. He said the fans were turning on at the proper temperatures, etc.
This morning I replaced all the plugs/wires to try and fix the problem. After about a 6 hour job, it was all done, and ran excellent, much better than before, smoother and no stumbling. Later today I took the Vette out for an errand (another 95+ degree day). After sitting at a stoplight for quite some time, the temperature got up past 3/4 and once hit 260. I took off from the light and it started stumbling really bad, worse than it has ever been. No power, running really rough through all the gears.
The other things I've been told that it could possibly be is the dreaded OptiSpark or possibly the water pump.
I'm trying to determine what I should do next. Your opinions/expertise is greatly appreciated!
Drifting
Joined: Sep 2003
Posts: 1,363
Likes: 7
From: 1994 LT1 Coupe 6-speed with FX3 & 2000 LS1 Vert 6-Speed with F45 Hunterdon County, NJ
PoloC4,
There can be many reasons for the stumbling, however, first things first -- you need to get the coolant temps down.
Suggest first reporting back the digital dash's coolant temperatures relative to the analog dash gauge.
Also, check in front of the AC condensor (in front of radiator) for debris and also between condensor and radiator (look in from the passenger side where the AC lines enter).
Also, check the 'air dam' (in front/below the radiator) -- if it is badly worn (or missing) it will not direct the air upwards into the AC condensor/radiator.
Don't forget that turning on the AC should turn-on both fans, and that most C4 owners use this 'trick' to keep the engine cool (e.g. should keep the temps around 200 degrees F in your 95 degree air temps).
Suggest also confirming your 'mechanics' observations of the coolant system fans (as it sounds like there may be something up) --
1. does the primary fan (driver's side) come on when the 'digital' dash coolant temp hits 228 degrees F?
2. does the secondary fan (passenger side) come on when the digital dash coolant temp hits 236 degrees F?
BTW, whenever the analog coolant temp gauge doesn't agree with the digital coolant temp gauge, the reasons can be as follows:
1. corrosion on the analog gauge's sensor plug -- on passenger side, about 1/2 way back from the front
2. old/wrong thermostat -- LT1 engines use a unique (at the time) thermostat which has an extra plate on the bottom -- often aftermarket thermostats are the incorrect size, do not open correctly (or enough) or are missing that plate -- which cause the back of the engine to be much hotter than the front -- often shows-up by the analog gauge being higher than the digital gauge.
3. old anti-freeze not preventing boiling/spot-boiling -- should be replaced every 2-3 years, and should use the old-style green anti-freeze (ethylene glycol with silicates) -- e.g. zylex or texaco brand
4. radiator cap (on surge tank -- pressure/black tank by windshield) is not holding in the pressure (should be an ACDelco RC27 - 15 psi).
There can be many reasons for the stumbling, however, first things first -- you need to get the coolant temps down.
Suggest first reporting back the digital dash's coolant temperatures relative to the analog dash gauge.
Also, check in front of the AC condensor (in front of radiator) for debris and also between condensor and radiator (look in from the passenger side where the AC lines enter).
Also, check the 'air dam' (in front/below the radiator) -- if it is badly worn (or missing) it will not direct the air upwards into the AC condensor/radiator.
Don't forget that turning on the AC should turn-on both fans, and that most C4 owners use this 'trick' to keep the engine cool (e.g. should keep the temps around 200 degrees F in your 95 degree air temps).
Suggest also confirming your 'mechanics' observations of the coolant system fans (as it sounds like there may be something up) --
1. does the primary fan (driver's side) come on when the 'digital' dash coolant temp hits 228 degrees F?
2. does the secondary fan (passenger side) come on when the digital dash coolant temp hits 236 degrees F?
BTW, whenever the analog coolant temp gauge doesn't agree with the digital coolant temp gauge, the reasons can be as follows:
1. corrosion on the analog gauge's sensor plug -- on passenger side, about 1/2 way back from the front
2. old/wrong thermostat -- LT1 engines use a unique (at the time) thermostat which has an extra plate on the bottom -- often aftermarket thermostats are the incorrect size, do not open correctly (or enough) or are missing that plate -- which cause the back of the engine to be much hotter than the front -- often shows-up by the analog gauge being higher than the digital gauge.
3. old anti-freeze not preventing boiling/spot-boiling -- should be replaced every 2-3 years, and should use the old-style green anti-freeze (ethylene glycol with silicates) -- e.g. zylex or texaco brand
4. radiator cap (on surge tank -- pressure/black tank by windshield) is not holding in the pressure (should be an ACDelco RC27 - 15 psi).
Last edited by theadmiral94; Jul 18, 2009 at 09:52 PM.
Instructor
Joined: Sep 2004
Posts: 216
Likes: 4
From: Linton Indiana
After you get the cooling checked, if the problem persists you might look at the spark plugs to see what color they are inside. They should be a light tan. If they are black or white you probably have a fuel system problem (too rich or too lean). You might take a real hard look at vacuum lines for a leak, also.
