Assessing 1991 GMC Jimmy iuuse

[RJB44]

My 1991 Jimmy has had some issues as of late. The motor seems to misfire and backfire under load. I wanted to eventuate some things this weekend, hoping to maybe correct the issue through process of elimination. What are some things I can check on my own and perhaps inexpensively replace (e.g. plugs, wires ect). If I can't resolve the issue on my own through some basic diagnostics, I will have to take it to a garage next week.

The backfire and sputter doesn't happen consistently. I will be going down the road then all of sudden it hits, the truck will buck on once or twice then proceed as normal for a while. It happens both at low speeds and highway speeds.

350 EFI (stock) - I wouldn't consider myself advanced, but a little better then average when it comes to tinkering with my truck.

Appreciate any help, at least to get me started.

 

[rebelgregory]

Sounds ignition-related. Look for arcing wires etc first. Spray em down with windex, start the truck in the dark and you'll usually see/hear the arc if that is your problem. If none, move onto cap and rotor. You can clean all the contacts and the problem may go away (temporarily). If not, check the contacts on the ecm for corrosion.

Wouldn't hurt to check the plugs, but usually wouldn't cause this issue. A cracked insulator would be more consistent. Good Luck.

 

[Jessie James]

Proper problem diagnosis:

1. Pull codes using a paperclip to your ODB port.

2. Review items which are broken.

3. Replace required parts.

Improper problem diagnosis:

1. Replace many parts, spending hundreds of dollars in the process, while never fixing the problem. Wonder why truck is still broken. Repeat. Drain checking account.

 

[rebelgregory]

1. Replace many parts, spending hundreds of dollars in the process, while never fixing the problem. Wonder why truck is still broken. Repeat. Drain checking account.

Diagnose then fix. All the tips I gave you are free, but I forgot to mention pulling your codes. However, sometimes, with a random misfire your codes (if there are any) can be misleading.

Also, you mentioned backfiring. This can be caused by unburned fuel (from a misfire) entering a hot exhaust manifold and auto-igniting. However, it can also be caused by poor timing, but bad timing usually has more consistent symptoms. But in case you're checking things and coming up short, check timing just to eliminate it as a cause. Don't forget to disconnect the brown wire.

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