1989 GMC Jimmy Engine Backfires under load

JSPIRITWALKER

MEMBER
1989 GMC JIMMY
V8
4WD
AUTOMATIC
180,000 MILES

I have a full size Jimmy with a 350. I had a cracked head and blown head gasket so I replaced the cracked head, had a valve job done and put it back together. I was careful not to mix push rods and rocker arms and such. I observed all the torques specs. I replaced the distributor cap, plugs, and wires. I changed the factory exhaust to headers (did not put O2 sensor in headers) and did away with the smog pump hoses. When I crank the engine it seems to run nicely, even when I rev the engine, but when I drive it with a load on it it backfires thru the exhaust and lugs down. I've played with the timing from 0 (setting called for on the engine compartment) to 8-10 BTDC. I replaced the fuel filter but that had minimal effect, if any at all. It's nice to have something that sounds good in the driveway but it would be nicer to have it run down the road.

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

Backfire In The Engine Exhaust System

Step 1 - Engine timing is to be set at a specific degree in relation to the crankshaft of the engine. If this timing becomes miss aligned it can cause low power, poor gas mileage, engine ping and backfiring. To check the engine timing locate the number one cylinder of the engine. This number one cylinder will be the forward most cylinder of the engine. On straight 4 and 6 cylinders engines it is pretty obvious, but on "V" style of engines like the v8 it is a little more tricky. The cylinders heads on every "V" style of engine are offset from one to the other. One cylinder head is more forward than the other. The most forward cylinder head is the side of the engine were the number one cylinder is located. The forward most cylinder is the number one cylinder.

Next, locate the crankshaft timing marks, most engines timing marks are at the front of the engine on the harmonic balancer. Near the balancer there is a small pointer or scale to align the balancer mark too. When the engine is running this gauge or pointer should align with the mark on the balancer. When the engine is running the timing light will strobe and illuminate the balancer mark as it spins. To adjust the timing loosen the distributor and turn slightly one way or the other to achieve the desired setting. Once an adjustment has been made re-tighten the distributor hold down bolt or bolts. Some engine's timing marks are located at the rear of the engine on the flywheel. Before you begin start and run the engine until warmed to operating temperature. Connect the positive and negative leads of a timing light to the battery. Observe the timing tag either on the engine or engine compartment for the timing specification and procedure. If the specification and instruction cannot be found consult a car repair manual.

Step 2 - Some engine exhaust systems have an air injection system designed to help the complete consumption of exhaust gasses. This added air is only in operation during power mode. If the air injection check valve or sometime referred to as a "gulp valve" fails it can allow extra air into the exhaust system causing system internal and external backfiring. To check your systems valve remove it and check air flow. Air should only travel one direction hence the name: "check valve". If air travels both directions the valve has failed and needs replacement.

Step 3 - The exhaust system of your car is designed to remove poisonous gasses from the engine to the rear of the vehicle. If something in the ignition system is intermittently failing it can cause unspent fuel inside the exhaust system to ignite when the ignition system returns to operation.