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97 Bonniville multiple misfires

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97 Bonniville, 200k miles, 3800 motor. It's a friends car.

He was driving his daughter to B-ball practice, stopped at a stop sign, and said the car started running real rough, like it dropped a couple cylinders. Pulled into the parking lot, it would crank but not start. I towed him home.

The code reader said "Multiple misfires"

Putting a socket and ratchet on the harmonic balancer bolt I can turn the motor over, but I'm not feeling resistance like I think I should when a cylinder is on it's compression stroke. The cams turning. I took off the oil fill cap and I can see the set of valves going up and down.

For some reason my guts still telling me it's a timing thing. Do these motors have the nylon/plastic tipped cam gear like others do? I'm not real familiar with the 3800 motor but I've had other high milage motors, mostly chrysler, with the nylon/plastic tipped cam gear stripp the tips and the timing chain jumps.

If it was something like a cam position or crank position sensor would the code reader tell me this?

Any thoughts?

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My first thought would be crank sensor, next I'd check for a possible bad ignition control module.

Not to say it isnt the timing chain but I've never had to do one on a 3800 so I dont think its all that common of a failure.

Does it sound normal when it cranks?

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Soooooo, I'm here at work overhauling a multi-gabillion dollar machine, thinking aboot my friends 800 dollar car.

I don't have fancy diag stuff in my garage, just my brain which is mostly gone. If I pull a couple plugs, ground them out, and crank over that eliminates the coil pack. However, if the crank postion sensor is bad, will it still spark?

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No. There will be no spark and no injector pulse.

If there is injector pulse but no spark than its likely the module.

I like to watch the ins and outs at the connector for the module with a lab scope. Takes about three minutes to see what's not working.

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Got home from work, grabbed a couple of beers and headed off to my friends house. We cranked it over for a while and it sounded even, then it tried to pop every now and then. The try to fire was rare, but it happened.

Pulled the easiest 3 plugs, grounded them, and cranked it over. They were sparking big time. The plugs also had a good bit of fuel on them.

Seeing this he says," It's got spark, it's getting gas, I'm taking the front of the motor off and checking out the timing."

I'm not going to stop him but it makes sense. Are we overlooking something?

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unplug the MAF sensor, hold the gas pedal to the floor, crank it over. Did it start?

Dunno. After he decided his timing chain was junk, and we had another beer, I went home. We work together so I'll see him in the morning and find out if he actually took stuff apart.

I'm curious tho. Why unplug the MAF sensor? Where are you going with this? Help me out here, man.

Scott, I gave it a shot of ether, and it hit for a second and that was it.

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when the MAF wigs out it will cause all kinds of issues. Since its data is used to control fuel among other things it can cause the engine to run lean or rich. When you said it was popping the first thing that popped into my head was the MAF.

The other possibility is that the fuel pump failed. DId you check fuel pressure?

I don't remeber what intake the 97 has but did you check the coolant level? did you pull any of the plugs on the backside? was there signs of coolant on the plugs? A ruptured intake plenum will flood the engine with coolant and cause it not to start. Typically they will not crank either?

In order to pull the timing cover you will need to pull the crank pulley. You will need a specific bolt(s) with a specific length that will thread into the crank pulley. A harmonic balancer puller can then be used to pull the crank pulley off. It is not difficult, just much easier with the right bolts.

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MAF measures the air coming into the engine, the pcm uses this info to figure out how much fuel to add to the air for proper combustion.

If the input is garbage than the out put is worse garbage.

With the MAF unplugged the PCM will resort to other inputs to "guess" the correct amount of fuel to the air.

hope that's simple enough. (darn I was to slow oh well)

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