Very Odd Back Fire Problem

[John Deere 4240]

Hey Guys,

I have a 99 c1500 Sierra with 320,*** miles on it now. Drove it hunting on turkey day and it ran fine, started it back up once we were done and it started running like crap. The whole truck shakes at idle and starts backfiring a lot when its under load. Seems to still have decent power, as in will still do 60, and uses all the gears.

I checked the slack on the timing chain by turning the crank till the rotor bug turned, then turned the crank backwards and the rotor moved almost immediately. So not enough slack for it to jump. Turned number 1 to TDC and the distributor was showing its in the right spot. That was also tight enough that it didn't turn on me either.

So I then hooked it up to a Bosch scanner at my local oriely's. That showed that at idle my timing was advanced to 25-30 degrees. Once I gave it some throttle it RETARDED to 5-10 degrees. That's backwards from how its supposed to work I believe. New Camshaft sensor read the same numbers on the same scanner. Double checked that the plug wires were not crossed either.

so what we working with here...new ac delco distributor, new nkg spark plugs, new ac delco plug wire, and new coil, (all replaced about 10-15 thousand miles ago) New Cam sensor and knock sensor. everything else is either old or ancient lol

Codes being throw: intermediate MAF sensor reading (20,000 old MAF), Multiple random misfires, manifold barometric pressure low (MAP), intermediate throttle position sensor (old as hell sensor).

Ive heard talk that this random $hit storm of sensor errors and timing issue could be an PCM failure.

Thanks for any insight you guys have on this.

 

[SAATR]

New cap and rotor and try again. I chased my tail for days trying to run down a P0300 and rough running because I automatically eliminated my less than two year old ignition components. After moving injectors around, testing and swapping various sensors, and pulling out my (thin) hair, I put a new cap and rotor on it and cured all that ailed her. These weren't el cheapo components either, but quality BWD replacements.

Edit: As a side note, misfires and/or knock will cause the ECM to pull timing, so that makes sense. If it misses badly at idle, that will cause low vacuum. TPS shouldn't cause a misfire at idle.

 

[twomanymontes]

As for the Mass air flow codes, Are you running a K&N type filter? make sure you didn't over oil it. That oil gets on the sensor wires and will create all types of issues. Try cleaning it with mass air flow cleaner and gently blow air across it before reinstalling it.

 

[slowburb]

Any chance it was wet, humid, rainy, on turkey day? I'm leaning towards moisture in the cap as the cause of backfire and idle quality.