Mailbag: P0300 Intermittant Misfire

[Bruce Nunnally]

Got this email from a Caddyinfo Reader about their 1999 STS:

I’m being assaulted by the nightmare P0300…

99 STS standard stuff 90k miles, new plugs, new wires, cleaned EGR, 80k new fuel filter

In Sept before vacation to Kazakhstan, engine developed a distinct dropped cylinder miss at idle, very intermittent. Felt like ignition module.

Return from KZ late Oct and it ran fine. Drove 100+ miles not a hint of a miss. Service car with oil change and Techron FI cleaner (concentrate-2 bottles) On the way from picking up wife & child from return trip at airport – BAM engine stutter, decreased timing and SES light

90k – OK new AC plugs and good Silicone NAPA wires, stared at ignition module for a long time – Fought one in a 91’ cutlass 3.1ltr years ago

Throwing Codes: U1064, U1128 (air door?), B1343, P0300 (Arrrggg!)

I hoped I fed it too much Techron and ran the gas out, but alas I’m staring at something much different.

Codes:

DTC U1064 Loss of Communications with DIM

DTC U1128 Loss of Communications with IRC

DTC B1343 Air Inlet Door Movement Fault

DTC P0300 Engine Misfire Detected

[Ranger]

The Guru always said too much injector cleaner can eat the insulating varnish off the fuel pump & injector windings. Might not be the cause, but could add to the problems.

Perhaps a bad coil? Listen to the coil with a stethoscopes for an arcing sound or pull them and see if the contacts look burned.

[Cadillac Jim]

The IRC is the Integrated Radio Chassis. The two network codes may mean that there is a loose connector under the dash, or that other messages are throwing off messaging synch in the network (i.e., nothing).

The air inlet door is the HVAC door that controls inside/outside air. If it's sticking, there may be something amiss under the dashboard behind the glove compartment, such as a wiring harness shoved against the mechanism, or a vacuum line could be off the vacuum distribution box, or off the vacuum servo that operates the door.

The code P0300 is set, according to my 1997 FSM for Northstars, by insufficient throttle response below 45 mph, off-idle while getting back on the gas, and the miss isn't consistent or bad enough to set a code for a particular cylinder. The troubleshoting process says to look for a spark plug or coil problem. One thing it says to do is to wet the ignition wires to see if that reproduces the problem.

If it happens at idle, you should be able to hear it under the hood in a quiet environment unless it's a bad plug or coil. At night, you may be able to see it, or at least see the flash if it's not in the line of sight.

My money is on either a poor moisture seal somewhere with the new wires, or a bad or poorly installed plug. Other possibilities are a loose connector on a fuel injector or a breaking down coil.

[BodybyFisher]

I would do an easy compression test on each cylinder to determine if you have a mechanical problem like a burned valve to eliminate this.

Has the intake manifold been off for any reason? Starter replaced, etc? A vacuum leak will cause a mis fire.

Has the fuel rail been off?

If you trashed an injector with the two bottles of Techron, it will have high resistance, check each injector's resistance and see if one sticks out, DO NOT physically clean them if they look dirty, you will really mess them up.

See the misfire articles under TUNE UP here

http://www.autotap.com/technical_library.asp

[Logan]

Hmmm....I just did mine today. + the wp one too. Both were original.

A bad serpentine belt can cause po300.

Overcharged AC.

Bad belt tensioner.

[KHE]

Hmmm....I just did mine today. + the wp one too. Both were original.

A bad serpentine belt can cause po300.

Overcharged AC.

Bad belt tensioner.

How would a bad serp. belt cause the misfire code?

[Logan]

The way the PCM detects misfire is by crankshaft speed. A misfire slows down the crankshaft a little bit, and the PCM can detect this change in speed.

So a bad belt or accesorry that slows down the crankshaft, can create a false po300.

I myself, have seen a bad belt create a po300. I didn't believe it the first time, but I have seen it happen.

As for my belts. Mine was just P.M. No po300.

Also, here is some more info from the GM service manual:

Check for the following conditions:

• Incorrect CKP System Variation data stored in the PCM--Run the CKP System Variation Learn Procedure using the scan tool.

• System grounds--Ensure that all connections are clean and properly tightened.

• A damaged accessory drive belt or driven accessory--This may cause engine load variations sufficient to set a misfire DTC. Refer to Symptoms - Engine Mechanical in Engine Mechanical.

• The fuel pressure--Perform a fuel system pressure test. A malfunctioning fuel pump, a plugged filter, or a malfunctioning fuel system pressure regulator will contribute to a lean condition. Refer to Fuel System Diagnosis .

• The fuel injectors--Refer to Fuel Injector Coil Test .

• Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• An extended Idle--Excessive open loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. In order to determine if this condition is causing the DTC P0300 to be set, review the Freeze Frame/Failure Records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs during idle or during very low engine speeds and at engine coolant temperatures below 80°C (176°F), the condition described above is probably the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 g/s.

• Running the vehicle low on fuel--This DTC may set if the vehicle misfires while running out of fuel. If no misfire is currently present, but the History Counters indicate random misfire was once present, ask if the vehicle has been run out of fuel. If this is the case, no further repair may be necessary.

[swalker140]

Bruce - Thanks for the post! I'm finally able to log in as a registered member so I thought I would update everyone on the outcome of the original ailment. I'm a systems engineer, but not for Delphi. I thought surely the controller would be able to identify a bad coil module. Rule#1 Assume is to make an *smurf* of u and me. Research group says "some ignition module components are assembled in matched sets and differ from year to year." A quick purchase on eBay provided an ignition module assy from a 1996 ElDog. The D1980A module that I received was a 1103971 4818 with a set of D555 ignition coils. The module that came of my 1999 STS was a 1104044 98050 and the coils are a different assy number also. The 4818 module with the coils that it came with, cured the malady of P0300 and restored the Northstar to it's originally lube quietllness.

I cannot find any information about the differences in the ignition module components. Caddy Parts just has cross-references. Anyone have any ideas?

Thanks guys for the insightful posts! Keep up the good work!

I need some tips about motor mounts! I have been through two 1999 Black STSs and six motor mounts between the two. I was thinking my wheel/tire choice was affecting it by possibly increasing gear ratio as the 255/45/18 Michelins were shorter than OEM wheel/tire, but I have since swapped to 245/50/18 Bridgestones that are the same height as OEM and it does no good. Offset is good. Wheels are Mille-Miglia 1000s for Euro STS. I see no sense in increasing torque in this engine without some type of resolution in the motor mount arena.

Cheers!

[BodybyFisher]

Hmmm....I just did mine today. + the wp one too. Both were original.

A bad serpentine belt can cause po300.

Overcharged AC.

Bad belt tensioner.

I was actually going through SI over the weekend and saw a reference to serp belt causing a P0300, pretty amazing, who would have thunk it..