99 GMC Jimmy SLT HEADACHE

GMCNIGHTMARE

MEMBER
1999 GMC JIMMY
6 CYL
4WD
AUTOMATIC
91,000 MILES

Hi, I have a 99 Gmc Jimmy that im having some very irritating problems with. My truck only has 91000 miles on it and have certified service records where GM replaced the engine at like 8000 miles. My ses light is on throwing codes p0300 p0172 p0174. Very hard idle and start when the engine is warm but when its been sitting overnight and its cold its starts up great and runs like it should. Couple monthes ago I replaced the fuel pump ive changed the plugs and wires and the etc sensor. I have spark on all cylinders but the driver side cylinder closest to the firewall has spark but it doesnt change the way the engine runs when its unplugged or not. Also sometimes it gets a wierd popping noise in the exhaust and has some liquid spatter coming out of the exhaust. The gas mileage is HORRIBLE I need to know whats wrong with it so I dont tear all the accesories and the whole top half of the motor off for no reason PLEASE HELP. Thanks

1 Reply

CARADIODOC

MECHANIC
33,916 POSTS

Sounds typical of the injectors. While you could have a bad one, it is more likely they are mismatched. Chrysler is one manufacturer that has almost no trouble with injectors because they buy them from Bosch in matched sets. GM just grabs a handful out of the box and throws them in. Because they flow at different rates, you will always have a few cylinders running too lean and a few running too rich. That very often results in the random cylinder misfire codes that GM is famous for. Since you already made the dandy observation that one cylinder isn't contributing his fair share, you might consider swapping that injector with one of the other ones to see if the problem follows the injector to the new cylinder or if the same cylinder still has the problem. When one injector doesn't squirt the proper amount of fuel, the normal amount of air is still going through that cylinder. That unburned oxygen is detected by the oxygen sensor as a lean condition. The Engine Computer will request more fuel from either all six cylinders or from all three on that side of the engine, depending on the model and year. No matter how much extra fuel is added, there will still be that unburned oxygen. O2 sensors don't detect unburned fuel, just oxygen. Fuel mileage will be terrible too from all the extra fuel going in.

There is a place in Indianapolis that rebuilds injectors and sells them in matched sets. The fellow's name is Jim Linder; not sure of the name of the company. They buy used injectors too to keep up with demand. Many GM owners comment that their engines never ran so smoothly until they replaced the injectors.