GMC Repair: 1991 gmc sonoma, oil pressure gauge, oil pressure sender

Question
Mr, Eliassen,

After I received your advice, I took a look at the sonoma again and neutral cable was good. I have used a bottle of fuel injector cleaner. The truck now keep running with rough idling. When I unplug one of spark plug, it was covered by black char. One of my friend told me that it was caused by plugged up catalyc converter. Does this make the engine skip firing up in the right way? Also, the battery seems to be bad on the truck. Does the battery has anything to do with idling? The computer was replaced recently and I think it is o.k. except for the oil pressure gauge is shot all way up all the time. Please give me your an expert advice on this truck. Thank you.

Answer
Hi,Blade

A restricted exhaust such as bad catalytic converter can give you a rough idle,but ive seen it more so when you drive and accelerate it will lack power and missfire,i would try and see if the EGR valve may be stuck open,remove the EGR valve and inspect for a restriction,the valve when removed looking at the bottom,then pintle should be closed.
A bad/failed battery wouldnt cause a missfire unless the voltage when the truck is running is below 8.5 volts,so i would rule that out as the cause for the missfire.
The oil pressure gauge could be 2 things that are common failures,one is that the oil pressure sender/switch is faulty and needs to be replaced,another is the gauge itself is faulty and in order to replace that you need to replace the cluster/IPC.
 Check the EGR valve,also make sure the engine has been tuned up if it needs it,new cap rotor plugs and wires,also check the ign coil,make sure the spark is good,it should have a blue spark,not yellow in color,if it is yellow in color it could be a weak coil.


ED