Runs rough

[sgth50]

I have a 97 Blazer. It runs rough, misses, and bucks when cold. After about 1-2 mnutes, it runs much better, but it is still running way to rich, as I can smell it and see a significant gas mileage reduction. I replaced the plugs and wires and no change in performance. (Boy that 2nd plug on the drivers side was a pain in the butt.) The plugs smelled and look like you would expect with a vehicle that was running rich.
The engine light was on in the past, and then the battery died. When the battery was replaced the engine light is now off. Are codes still in there somewhere? Cant figure it out!! This car has been a pain.

Help!!

[ctwright]

Probably isn't it but I'd start by checking the air filter.

[old_master]

Originally Posted by sgth50

.....
The engine light was on in the past, and then the battery died. When the battery was replaced the engine light is now off. Are codes still in there somewhere? Cant figure it out!! This car has been a pain.

Help!!

1996 and newer vehicles are equipped with OBDII and DTC's remain in the history file until the DTC's are cleared with a scan tool. The system was designed to retain DTC's in the history file so that in the case of battery failure, the technician can still retrieve DTC's.

Check fuel pressure and leakdown. Key on, engine off and cold, fuel pump running, pressure must be 60psi to 66psi and remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Post your results.

[sgth50]

Had to give the blazer to a friend because it is over my head. He said at 40mph the fuel pump pressure is 55-60psi. He has not given me the numbers yet that you asked for.

He replaced the fuel filter......original filter, and could barely blow through it. Now it runs better at times, but when it starts missing and bucking, it is worse than before. However, he found that if you turn the truck off, and then back on it will run 95% better. When it misses, he said it is usually missing on all cylinders...according to the scanner.

I will get the numbers you wanted hopefully tonight.

[sgth50]

Check fuel pressure and leakdown. Key on, engine off and cold, fuel pump running, pressure must be 60psi to 66psi and remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. Post your results.[/quote]


60psi cold
35psi after 3-5 minutes

[old_master]

Leakdown is excessive, probably what's causing your problem. Provided there are no external leaks, the leakdown could be from under the plenum, but most likely it's in the tank. The pump needs to be isolated from the system just after pressurizing the system to determine where the leak is.

[sgth50]

I agree, but why .....when it runs bad, I still have 57-60 psi going through to the system. I do not understand why sometime it runs fine with that pressure and sometimes it runs terrible. I would think leakdown would effect start up, and mine starts without alot of cranking. If I turn off the vehicle when it is missing on all cylinders, and then start it up again it will run nice. Just seemed electrical to me, but I am an idiot when it comes to this car.

[old_master]

GM does not publish any "engine running" fuel pressure specs for the CSFI system, there are too many variables involved. All pressure tests are done with the engine cold and not running.

Are there any DTC's in memory, (pending), after it runs poorly? If so what are they? When the engine is cold, and before you start it, check the ECT, (Engine Coolant Temperature), sensor and the IAT, (Inlet Air Temperature), sensor. They should both indicate the same temperature. Are you using an oiled air filter? If so, excess oil from the air filter can contaminate the MAF, (Mass Air Flow), sensor. If that's the case, carefully clean it with a solvent that does not leave a residue. Another thing that might be worth checking is the ignition switch. 1997 has known issues with it.

[autogas01]

Originally Posted by sgth50

I agree, but why .....when it runs bad, I still have 57-60 psi going through to the system. I do not understand why sometime it runs fine with that pressure and sometimes it runs terrible. I would think leakdown would effect start up, and mine starts without alot of cranking. If I turn off the vehicle when it is missing on all cylinders, and then start it up again it will run nice. Just seemed electrical to me, but I am an idiot when it comes to this car.

I have had that problem with a 96 merc sable. missing on a few cyls. then i flip the ignition switch off and back on and it runs great. The same thing is happening with my 97 t-bird now.
And it will not idle after starting in cold outside temps.