engine hesitates

06-14-2004, 03:32 PM
i've got an 88 gmc s-15 jimmy. i'm not sure what's going on.

it has a rough idle with rpms jumping around a bit and rpms jump even after acceleration. at any speed rpms drop to nothing and the car hesitates for a second and then recovers.

i don't recall any noises and there are no smells or anything like that.

any ideas where to start?

06-22-2004, 12:23 AM
One place to start is the vacume lines. They could be old and sucking air. The easiest way to find the bad lines is to put gas into a spray bottle, start the engine, and lightly spray gas on the lines. Avoid spraying gas on the distributor because you dont want to start a fire. When the idle jumps you've found a leaker replace the lines. Another thing that could be wrong is the throttle positioning sensor could be going out.

08-30-2004, 05:49 AM
That is also a symptom of the ignition module failing. On a vehicle that age I would think also that the electrical connectors are in need of cleaning. You should check the chassis ground on the O2 sensor - there's likely to be none, hence the ECM will get a false signal from it. I think if it does get a false signal it will be "mixture too rich" and will cause the ECM to lean the mixture.

You should check the fault codes as well; there may be an EGR problem. The electric vacuum valve is prone to being very leaky, possibly causing the mixture to go too weak temporarily, hence a stall.

08-30-2004, 01:47 PM
my car decided not to start a few days ago so...

i changed the spark plugs, the o2 sensor, and cleaned the carb really well and it still won't start.

i've been told timing could've slipped and distributor might need to be rebuilt... any other suggestions?

08-30-2004, 06:25 PM
If there is no spark at the plugs. Fit a plug to the centre lead. Place it on the engine so that the threading is grounded, DO NOT HOLD WITH YOUR HAND - HIGH VOLTAGE. If need be, use heavily insulated pliers or a piece of wood to keep it in place. Have a friend "start" the engine. Watch for a spark jumping the plug gap. If none then the coil, ignition module or pickup coil has failed. Of the three, the ignition module is the most likely to have failed. I am assuming the distributor cap and rotor are in good condition when I say this.

Ignition Coil Test

You need to use a multimeter.


CP o <- coil HT post

1 = 3 = <- electrical connector
2 = 4 =

1,2 Front set 3,4 Rear set of connectors


A On high scale, resistance between 1 and chassis (battery neg) should be infinity (a "1" on digital meters).

B Between 1 and 4 a low reading or zero ohms

C Between 3 and CP a high reading but not infinity.

Anything else is a failed coil.

Ignition Module

The ignition module is electronic and for some reason GM decided it should be mounted in a place which is inhospitable to electronics - the distributor. Electronics cannot stand heat and HT for long, so they fail, in part or whole.

You'll have to remove the distributor to change it. Clean away any grit and grease from the base of the distributor. Disconnect the wiring to it. The distributor is held in place by a 1/2" AF bolt and goats foot clamp. Mark the position of the distributor and the rotor. When lifting the distributor out, the rotor will rotate about 30 degrees. Mark that position too, for when you refit it, all has to end up in the place it started from. There is a hex section shaft loosely attached to the base of the distributor. It sometimes comes out with the dizzy, so shake the dizzy as you lift to make the shaft drop back.

The ignition module is fitted to the base of the dizzy by two 11/32" AF machine screws. Remove and clean the old grease off. The new module comes with a satchet of heat transfer grease. Apply ALL of it to the module base plate and then fit to the dizzy.

Reconnect the connectors to the dizzy and the spark plug to the centre HT spin the dizzy by hand, rotor going clockwise and check for a spark (ignition on). Refit the dizzy if ok.

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