99gtp - P0171 & P0306

[slimfatboy]

Hello everyone -

First off, my 99 GTP is starting up and in the cold weather is idling very rough. It will start off around 1000 then fall to 400-500 and then jump up to 2200-2500 and then back down to 400-500, occassionally stalling. In gear, it will idle right within 1000 +- 200 as it revs up and back down. Once it warms up, it does better but not as smooth as it was before.

I replaced the MAF recently (2wks) which cleared up another code it was throwing as well as the IAC and the front plugs. (Rears will have to wait until the shelving is hung in the garage again after falling.)

TIA,
Ted

[tblake]

Might want to check out that IAC motor, and Im sure the bad rear 3 plugs can be contributing to things also.

Get the codes read at autozone and let us know.

[slimfatboy]

Do you have any specs to check the IAC valve to? I read for some older models a specified resistance measurement that I can do but it was for an older model grand prix.

The plugs may not be helping, but the front three appeared pretty good and not what I have seen causing problems in the past.

The codes are P0171 and P0306, system too lean and cylinder six misfire.

[BNaylor]

Worrying about the front bank plugs won't help resolve your issue especially since the P0306 misfire is in the rear bank.

Start by resolving the P0306 misfire DTC (rear bank cylinder #6) before the P0171 issue. Most likely the respective spark plug, ignition wire or coil pack.

[tblake]

wait a minute bob, PO171 (system too lean)? along with a PO306? I might suspect here a problem with a fuel injector. Mainly becuase if the injector were firing and the cylinder were not getting spark, would we not see a misfire code along with a system too rich code? But we see the exact opposite, system lean and missfire. Meaning maybe the cylinder isnt getting fuel. Am I wrong?

[BNaylor]

Originally Posted by tblake

wait a minute bob, PO171 (system too lean)? along with a PO306? I might suspect here a problem with a fuel injector. Mainly becuase if the injector were firing and the cylinder were not getting spark, would we not see a misfire code along with a system too rich code? But we see the exact opposite, system lean and missfire. Meaning maybe the cylinder isnt getting fuel. Am I wrong?

Sorry minute's up

Technically the OP should not be getting a P0171 DTC in conjunction with a P0306 or any misfire DTC and that includes any possible fuel injector DTCs. No misfire or fuel injector DTCs is a prerequisite for P0171 to set.

Per the GM FSM for diagnosing P0171 - Fuel Trim System Lean:

"If any other DTCs are set, diagnose affected DTCs before proceeding. If no other DTCs are set, go to next step."

In other words diagnose and resolve all other DTCs before working on the P0171 DTC since many can affect this particular DTC. There is a laundry list of issues that can can P0171 which includes fuel injectors but I would look at a possible secondary ignition system problem first causing a misfire condition since it is the easiest thing to do and rule out. Then go to other possibilities like fuel, ............etc.

[tblake]

[quote=BNaylor]Sorry minute's up [quote]

HA HA!! your too funny!

[slimfatboy]

I wasn't so much worried about the front plugs but from their condition and length of time in service, I was speculating the condition of the back plugs. I did change them out today and as expected, they all looked good. At the time the plugs were changed, the plug wires were changed as well, not to say that one couldn't be faulty.

My thoughts were that it could be an injector as well. I planned on just unplugging one at a time excluding #6 and listening for a difference and then pulling #6 and checking for the same difference in sound/running.

I think I read that to check out the coil, just swap it with another one. So, I will continue on and do that and erase the current DTC's and see what happens.