Oh the trouble with engine codes...

[matttahoe53]

So yesterday as I was heading home from work my 99 tahoe started acting up. The truck would lose power under load, the engine was knocking, and under "harder" acceleration the exhaust would backfire on the passenger side. So I get home ok and on the way stop at autozone to have my codes read. This is what I got.

P0101
P0102
P0300
P0327
P0171
P0174

So I get home and did some research in preparation for today. First up I check the maf sensor. I clean it reinstall it and no change. Head out to autozone to get a new MAF sensor, install it, truck ran slightly better but still this was not the fix. Next I checked the fuel pressure. With just the pump on it held steady at 54 psi. I turned the engine on revved the engine, the pressure would jump to 60-62 psi and settle back in at around 54. Turned everything off and the fuel pressure remained constant. So, I don't think it's a clogged filter or bad fuel pump. The last thing I did was pulled the plugs of the passenger side since that seemed to be the side that was missing and is the only side that backfires (I have a true duel exhaust). From front to back, the first two plugs looked fine, the third was a little fouled, and the fourth was nasty. So I head back to the store to buy new plugs. When I came back I decided to check my header bolts. The front ones were tight but the rear ones were loose with the furthest bolt to the back needing hand tightening to get it started. I throw the new plugs in and while the truck has definitely smoothed out its idle, under load it still bogs down and back fires.

I am at a loss now. My next thoughts are spider issue, o2 sensor issue, or a clogged catalytic converter. What are some other possibilities?

 

[badazzbulldog]

check the cap and rotor

 

[matttahoe53]

check the cap and rotor

I don't know why I didn't think of this. That's an easy check.

 

[Big98z71]

My bet would be coil and/or ignition module. My 93 did the same thing. Module was dead

 

[matttahoe53]

I forgot to add that when pushed there would a sulfur smell from the cat.

My bet would be coil and/or ignition module. My 93 did the same thing. Module was dead

What is the best way to test the icm and the ignition coil?

 

[matttahoe53]

Update... I replaced the distributor cap and rotor. Before I remove the rotor I checked the play in it and found no slop with the rotor. This improved things slightly because I noticed that I only had one screw on the distributor cap. I am still getting backfires on the passenger side exhaust. I got the infrared temperature scanner out to check the cats and each individual header. The drivers side cat measured 450f coming in and going out. The passenger side was around 750f in and 800f out. Each header pipe was around the same temperature except for cylinder number two which was only at 180°F at operating temperature, half the temperature of all the other cylinders.

 

[matttahoe53]

I used an inline spark plug tester and am getting a signal to the plug.

 

[matttahoe53]

Well the problem is fixed. I am not sure what fixed the issue, but for future reference this is what I would do before buying a single part or taking things apart.

1. Use an inferred heat gun when the engine is at operating temperature to check the catalytic converters to see if the temperature is the same going in as coming out as well as compare the temperature of one cat to the other. This will give an indication as to which side you may be having an issue. If you have headers you can compare the temperature of the exhaust gas of each cylinder.

2. Check for vacuum leaks. I inadvertently created one when I removed a bolt to pour seafoam into the engine. The bolt was in the line that used to go to my brake booster, I have hydroboost so the line is no longer needed.

3. Use an inline spark plug checker to make sure a spark is getting to the plugs. I had a fouled distributor cap terminal and a slight crack in a spark plug wire.

4. Check the fuel pressure heading to the intake.

I think if I had followed these steps I might have avoided buying a maf sensor. I may have needed one anyway as I did notice an improvement when I replaced it. Hopefully this can help someone else down the road.

 

[Fallsey]

Knock sensor will cause that bogging down issue too


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[matttahoe53]

Knock sensor will cause that bogging down issue too


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The knock sensor is around a year old from when I completely flushed the coolant. Anyway, whatever I did has the truck sipping gas (relatively speaking) and roared down the highway on my commute to cincinnati and back today.