1996 Misfire

Tex18 · March 15, 2008

I have a 1996 GMC 1500 with the 5.7L engine and I am trying to find the cause of a cylinder 4 misfire. It started throwing a misfire code and I found a few spark plug wires that were loose and since it has happened before I decided to put new plugs and wires on. It is still misfiring, as indicated by the code 0304 and the flashing of the check engine light. The light only flashes on start up until the truck warms up then it stays on steady. I thought I might have gotten some bad gas (it started right after I got gas at a local station, with a day or two) so I have also ran a few cans of Seafoam through it with no luck. I then changed the distributor cap and rotor. Again with no joy. I am getting ready to do a compression check when the weather will allow me. I do however have a code reader that takes a snap shot of the engine date at the time of misfire. The only problem is I don't know what all of it means. I am new to working on vehicles ( I am jet mechanic by trade) and I am looking for any information from the data below to prevent just throwing parts at it at random and hoping to get it right. The truck has 192000 miles on it and according to the last person who owned it he had the top end rebuilt about 50000 miles ago. Up until two weeks ago the truck ran great. I had considered an injector problem but with the throttle body injection I figured that I would have more than one cylinder misfiring if something was wrong there. Any information helps. Thanks Jeremy

Here is the data that downloaded with the code reader

P0304 Cylinder 4 Misfire Detected

MIL ON

MisFire Monitor Complete

Fuel System Monitor Complete

Comprehensive Component Monitor Complete

Catalyst Monitor Complete

Heated Catalyst Monitor Not Supported

Evaporative System Monitor Not Supported

Secondary Air System Monitor Not Supported

A/C System Refrigerant Monitor Not Supported

Oxygen Sensor Monitor Complete

Oxygen Sensor Heater Monitor Complete

EGR System Monitor Complete

DTC for which Freeze Frame was Stored P0304

Fuel System 1 Status Open Loop

Fuel System 2 Status Open Loop

Calculated LOAD Value 5.10 %

Engine Coolant Temp 68.00 °F

Short Term Fuel Trim - Bank 1 -6.25 %

Long Term Fuel Trim - Bank 1 -3.13 %

Short Term Fuel Trim - Bank 2 -1.56 %

Long Term Fuel Trim - Bank 2 3.91 %

Intake Manifold Absolute Pressure 9.45 inHg

Engine RPM 895 rpm

Vehicle Speed Sensor 0.00 mph

Air Flow Rate Mass Air Flow Sensor 1.43 lb/min

ChevyTech007 · March 16, 2008

I have a 1996 GMC 1500 with the 5.7L engine and I am trying to find the cause of a cylinder 4 misfire. It started throwing a misfire code and I found a few spark plug wires that were loose and since it has happened before I decided to put new plugs and wires on. It is still misfiring, as indicated by the code 0304 and the flashing of the check engine light. The light only flashes on start up until the truck warms up then it stays on steady. I thought I might have gotten some bad gas (it started right after I got gas at a local station, with a day or two) so I have also ran a few cans of Seafoam through it with no luck. I then changed the distributor cap and rotor. Again with no joy. I am getting ready to do a compression check when the weather will allow me. I do however have a code reader that takes a snap shot of the engine date at the time of misfire. The only problem is I don't know what all of it means. I am new to working on vehicles ( I am jet mechanic by trade) and I am looking for any information from the data below to prevent just throwing parts at it at random and hoping to get it right. The truck has 192000 miles on it and according to the last person who owned it he had the top end rebuilt about 50000 miles ago. Up until two weeks ago the truck ran great. I had considered an injector problem but with the throttle body injection I figured that I would have more than one cylinder misfiring if something was wrong there. Any information helps. Thanks Jeremy

Here is the data that downloaded with the code reader

P0304 Cylinder 4 Misfire Detected

MIL ON

MisFire Monitor Complete

Fuel System Monitor Complete

Comprehensive Component Monitor Complete

Catalyst Monitor Complete

Heated Catalyst Monitor Not Supported

Evaporative System Monitor Not Supported

Secondary Air System Monitor Not Supported

A/C System Refrigerant Monitor Not Supported

Oxygen Sensor Monitor Complete

Oxygen Sensor Heater Monitor Complete

EGR System Monitor Complete

DTC for which Freeze Frame was Stored P0304

Fuel System 1 Status Open Loop

Fuel System 2 Status Open Loop

Calculated LOAD Value 5.10 %

Engine Coolant Temp 68.00 °F

Short Term Fuel Trim - Bank 1 -6.25 %

Long Term Fuel Trim - Bank 1 -3.13 %

Short Term Fuel Trim - Bank 2 -1.56 %

Long Term Fuel Trim - Bank 2 3.91 %

Intake Manifold Absolute Pressure 9.45 inHg

Engine RPM 895 rpm

Vehicle Speed Sensor 0.00 mph

Air Flow Rate Mass Air Flow Sensor 1.43 lb/min

ChevyTech007 · March 16, 2008

I don't know the reason for the above post. I hit a wrong button or something. Anyway, your problem sounds a lot like a intake gasket leaking vacuum. You can spray the sides of the intake with carb cleaner and see if it changes the idle, if it does not, pull the pcv valve out of the left valve cover and the hose out of the right valve cover that goes to the air cleaner. Plug both holes in the valve covers with your fingers and see if a vacuum develops. It should build slight pressure, if it's vacuum your intake gaskets are leaking vacuum internally. BTW, if your truck is a 96, you should have an "R" motor which does have individual injectors. Look and the eight digit of you vin #, if it's an "R" you have 8 injectors.

mtm700 · March 16, 2008

My brother's '98 currently has this problem on #4 also. I figured it was just the plugs, wires, cap and rotor. So I replaced them, he had 114,000 miles on the originals. Problem still existed. So I did a cylinder leak down test and found the exhaust valve sticking open. It runs awsome from 1500 RPM on up. I'm just waiting for him to bring the truck into the shop so I can get it fixed.

Tex18 · March 16, 2008

I don't know the reason for the above post. I hit a wrong button or something. Anyway, your problem sounds a lot like a intake gasket leaking vacuum. You can spray the sides of the intake with carb cleaner and see if it changes the idle, if it does not, pull the pcv valve out of the left valve cover and the hose out of the right valve cover that goes to the air cleaner. Plug both holes in the valve covers with your fingers and see if a vacuum develops. It should build slight pressure, if it's vacuum your intake gaskets are leaking vacuum internally. BTW, if your truck is a 96, you should have an "R" motor which does have individual injectors. Look and the eight digit of you vin #, if it's an "R" you have 8 injectors.

Thanks for the info, I checked the VIN and I do have the R code so I am going to check into the injectors. I also had a buddy look at the freeze frame data and looking at the fuel and the idle he thinks it might be a computer or fuel control problem. The freeze frame indicates that the idle is 895 but when I start the truck it is idling as 1100-1200 and he told me that is high for idle. Might be as simple as having the computer flashed at the dealer?

cammiller · March 16, 2008

If this happened after you fueled up, I would not be thinking a Calibration issue. Like the others that have replied, I would check carefully for the intake gaskets leaking. I cannot count how many I have replaced. How long has the truck ran since you put the new plugs in it?

Tex18 · March 16, 2008

i have only had the plugs in there for couple of weeks, but it was starting this before the new plugs and after the suspect gas. I have heard a lot of people say that the intake gasket is the likely candidate. I am just waiting for some better weather to look into it more.

Tex18 · March 22, 2008

Well I finally got the time to do the compression test and it appears all is well there. All cylinders were at 180 psi +/- 5 psi. But the truck is starting to run even rougher. The odd thing is one night I was driving home and it started running perfect, ran great all night and then next day was running twice as bad as original.

Tex18 · March 31, 2008

Well finally got a weekend to do a little bit of work to it. I pulled the upper intake off and checked the injectors and replaced cylinder 4 injector. Man it was a mess under there. I can't believe any of the injectors are working. There was a solid layer of sludge covering the whole top of the intake and the injectors. After pulling the injectors out, the ends around them were completely caked with sludge. Has anyone encountered this, is it normal. The truck has a lot of miles so I expected some, but not that bad. So I get the intake back on and it did not help at all. I am now looking at the intake gasket. Is there anyway to tell if the intake gasket is bad. I have tried the pulling the PCV out and checking for a vacuum on the valve covers and there is no vacuum and I tried the carb cleaner on the intake and no change in the idle. I avoided checking the intake gasket even though I was half way there because I need the truck during the week and have limited time on the weekend. Anything that I might be missing? I know a lot of people are saying intake gasket but I don't want to overlook anything simple before I invest the time and money going after the intake gasket. Could it be ignition module, the computer etc. And what could I expect to pay to take it to a shop for diagnostics, reprogram computer something like that should it come down to it.