89 SHO intermittent stumbling problems.

[89SHOdown]

I have an 89 SHO all stock with 114k mi. Over the last year it has developed an intermittent stumbling problem. At first it was very minor and usually happened when it was cold. It has now graduated to much more frequently even well after being up to normal running temp. It doesn't matter if I'm light throttling it or WOT. When it starts happening I can slightly back off the throttle and it will go away unless I increase the throttle even a little and it comes back. There are times when I can start the car and it runs perfectly until I turn it off and restart it. I've checked the fuel pressure and it comes up to 33psi when the ignition is turned on and settles at 30 when started and idling. It will go up to about 35 when I rev it. It will hold at 30 when I put it under a load. It will hold at 30 when I turn it off. Even after 30 minutes the pressure is still 30 psi. Car usually start quickly but on occasion it will crank 10 or more times before starting. This problem comes and goes but is occurring about 75% of the time now. Any help diagnosing this will be greatly appreciated.

 

[Devin]

I had stumbling issue and it was the CKS, but many things can cause it.

 

[zoomlater]

Are there any engine codes

 

[zak]

In addition to the suggestions above, plugs, wires or oil leaking into the plug wells can cause these stumbling issues. Have you washed the engine recently as water can get into the plug wells but is relatively easy to remove (except for cylinder 5)

 

[BaySHO Performance]

Are there any engine codes

...and let us know what you find. We can then go from there. Fuel pressure is fine.

If it has trouble starting when hot, that's caused by vapor lock in the fuel rails. Press the accelerator down some to give it more gas.

 

[shobote]

Another possibility, do you have an oiled air filter ? Mine acted that way years ago when I put a K & N air filter on. Turns out oil from the filter contaminated the sensor wire inside the MAF. Took the MAF out, cleaned the wire with q tip and rubbing alcohol, removed the K & N, and has run fine since.

 

[89SHOdown]

Are there any engine codes

No engine codes. Though I did get a cylinders out of balance message when running a cylinder balance diagnostics. Also after starting the engine after it had been turnoff for about 30 min, I attempted to run the cylinder balance diagnostics again and got a message stating that the engine temp was out of range. The gauge showed cold. Outside temp was about 60 degrees. After letting the engine idle for about 5 minutes it was able to properly read the temp sensor and allows me to run the cylinder balance diagnostics again. This engine has always run cold since I have owned it. During the winter it barely gets out of the blue unless I'm in stand still traffic then it climbs about halfway to the L in Normal. In the summer it runs below the L in normal and in stand still traffic it makes it's way to about the middle of the gauge. Never any higher.

 

[89SHOdown]

Another possibility, do you have an oiled air filter ? Mine acted that way years ago when I put a K & N air filter on. Turns out oil from the filter contaminated the sensor wire inside the MAF. Took the MAF out, cleaned the wire with q tip and rubbing alcohol, removed the K & N, and has run fine since.

Running a stock clean paper filter. I do remember the dirty MAF sensor wire issue. I'll try cleaning that. Thx

 

[89SHOdown]

In addition to the suggestions above, plugs, wires or oil leaking into the plug wells can cause these stumbling issues. Have you washed the engine recently as water can get into the plug wells but is relatively easy to remove (except for cylinder 5)

Have not cleaned the engine. I will check the sparkplug wells for oil. That could be an issue. I bought the car with 88k mi on it and was told that the plugs had been replaced at the 60 k servicing so I haven't looked at them. There are no obvious signs of oil leaks anywhere on this engine yet. It is dry on all external surfaces.

 

[BaySHO Performance]

I have had several cases where the spark plugs have been fully immersed in oil, but the car ran flawlessly. Never has oil caused a stumble or misfire in my experience.

I thought you said that you had already cleaned the MAF, but must have you mixed up with someone else. What happens when they are dirty is that it throws off the A/F ratio. Car runs fine under gentle acceleration, but bogs down with more gas.

The most likely cause of a cylinder out of balance during KOER is a clogged fuel injector. Might also be a dirty MAF. So first up is to clean it and rerun KOER.

The temp gauge reading way low is because of an old temp gauge sender. In the thermostat housing with a single wire coming off it. The gauge will read correctly with a new one. I have been using the Duralast TU25 from Autozone of late. However, their design has changed slightly. The post for the wire used to have rings on it to hold the push-on connector, like the oil pressure sender. This post is now threaded and the connector won't push on. Solution is to change the connector to a ring with a suitable nut and washer.