Question about 1999 GMC Jimmy

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Rough idle and misfire

Rough idle and stall. Distributor, spark plugs, rotor and wires all replaced, did not help.

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These trucks have a spider injector and all depending on the year it may have one central injector or 6 and then popets, these are lines that run to each injector, you will need to make sure the fuel pressure is good, low pressure will cause problems i think it is 62 psi key on engine off, and the other problem is the fuel lines going into the rear of the intake to feed the injector leak inside and cause rough idle and misfire, this is for the older one. the newwer trucks have the fuel lines external and you will see them on the top of the injector housing.

A good way to tell if the line is leaking , o and also the fuel regulator inside the plenium also leaks causing the same type of problem, to test you will need a fuel pressure gauge , hook it up ,turn key on and pressure will go up then turn key off and if pressure drops then you have a leak, could be fuel pump check valve but may be line or regulator, if you remove the throttle body and with a pen light look inside intake for leakage.or remove upper intake and do the same thing, better view this way.

Posted on Jan 23, 2013

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SOURCE: rough idle- code reader - multiple misfires after

do u hav the wires on the rite plugs-crosses are common--are plugs gapped rite--get a haynes manual for problems--dirty idle valve questionable

Posted on Dec 15, 2010

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Rough idle- code reader - multiple misfires after replacing- plugs - spark plug wires- rotor -distributor cap- filters


do u hav the wires on the rite plugs-crosses are common--are plugs gapped rite--get a haynes manual for problems--dirty idle valve questionable

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I am having ongoing problems with the truck miss firing and rough running. Timing is ok, replaced cap and rotor,coil. It still starts rough,idles rough and misses when at cruising speed,although it seems...


Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine's power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and electric rear defroster, for example).

When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust.

What causes a cylinder to misfire? Basically, it's one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad spark plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

"Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator. Low fuel pressure would affect all cylinders rather than an individual cylinder, as would most air leaks. A leaky EGR valve can also have the same effect as an air leak.

Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, it's likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant, it's likely the head gasket is the culprit.

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