Odd Misfire Conditions

[Twizzler09]

I've been hard at work trying to figure out why my '96 4.0 misfires for AGES.

If the following conditions are met, the next time I start the motor. It misfires like nobody's business and *will not* stop msifiring until the truck moves FORWARD. I can rev the hell out of it, even go backwards as far as I want (I put it in reverse and drove down the street backwards...when I came to a stop and let it idle... it was still missing). The strange thing is... it will only miss at idle. If I even tap the gas ever so slightly it'll quit....until I let it drop to idle again.

-The motor *MUST* reach full operating temp and hold it for at least 10 minutes.

-The motor must be allowed to cool for 20-40 minutes without starting.

- The motor must be started within one hour of being shutoff.

If those three conditions are met (with some variation....they aren't exact) the motor will start missing immediately upon starting. But as I said, it quits as soon as I go FORWARD. Going in reverse or revving it will not making it stop.

I'm completely out of ideas. Besides a possible problem with the fuel injectors maybe? wtf? help?

 

[Will]

All three indicators are directly related to the coolant temperature sensor. You have to warm the engine up thoroughly, and let it cool only partially. So the computer isn't sure if it is starting in the warm or cold mode. So maybe it's realy warm, but the sensor tells the computer it's cold and it's over fueling during warm-up. Something like that.

You have an EEC-V so I have no references covering that. The sensors should be the same though. ALthough there may be more of them on yours. Could be a temperature sensor in the airstream I guess.

 

[Bob Ayers]

Has it thrown a CEL? Have you scanned for codes to see what cylinder(s) are misfiring?

 

[Twizzler09]

Will, you may be onto something. I had it happen again earlier but this time I noticed a very strong odor from the exhaust. It was rather sweet smelling.

The only code that comes off the computer at ANY point is PO1443: Auxiliary Emissions control... which could be absoluetly anything. But no CEL is ever thrown. The emissions control has nothing to do with the current problem, I believe. I really don't know why its throwing that code anyway, as the emissions system seems to be perfectly fine.

 

[Bob Ayers]

Are you loosing any coolant? Antifreeze can have a strange smell in the exhaust. The 4.0L has been known to have the lower intake bolts loosen up, causing a vacuum leak, and also use oil, and antifreeze.

 

[Twizzler09]

I am loosing coolant, but I know the cause. Upper Intake. That, also, has nothing to do with the misfire. The smell goes away once the motor quits misfiring, which means the smell is directly related to the misfire condition. I am not losing Oil. I use 1/10th of a quart or less between oil changes.

Just to eliminate certain things, heres a list of known problems and wether or not they are related:

Code PO1443 Auxilliary Emissions Control (EGR related I think) (Non-related)
Leaky Upper Intake (Non-Related)
Squeaky Brakes (Non-Related)
Ball Joints (Non-Related)

a List of parts I have changed in the last year:

Spark Plugs
Plug Wires
Alternator
Fenderwell Starter Solenoid
Battery Connectors
Battery
Idle Air Control (IAC)
Catalytic Converter
Muffler
Front Rotors
Front Brake Pads
Front Wheel Bearings & Seals
Oil & Filter x4
------------------------------------------

Leaking Coolant would not cause a severe misfire. If it did, it would occur at all engine temperatures and under any condition, not just under the ones I listed, and in addition the mis-fire would remain after having moved the truck.

Lets not get off track here

Re-read my original post and look at the conditions that lead to the misfire. I believe Will is spot on by saying it could be coolant temp sensor related.

Now, on that train of thought, if the motor is trying to start in "cold mode" and dumping too much fuel into the motor, what does an overly "rich" Air/fuel mixture smell like?

 

[Combs Customs]

FYI, your Lower Intake is the one leaking. Only air goes through the upper intake, no fuel or coolant, just air. Bob Ayers is right. And coolant has a sweet smell when it burns, and you confirmed that true.

 

[baddis]

matt remember what i told you about the intake could be causing the problem

 

[Bob Ayers]

I am loosing coolant, but I know the cause. Upper Intake. That, also, has nothing to do with the misfire. The smell goes away once the motor quits misfiring, which means the smell is directly related to the misfire condition. I am not losing Oil. I use 1/10th of a quart or less between oil changes.

Just to eliminate certain things, heres a list of known problems and wether or not they are related:

Code PO1443 Auxilliary Emissions Control (EGR related I think) (Non-related)
Leaky Upper Intake (Non-Related)
Squeaky Brakes (Non-Related)
Ball Joints (Non-Related)

a List of parts I have changed in the last year:

Spark Plugs
Plug Wires
Alternator
Fenderwell Starter Solenoid
Battery Connectors
Battery
Idle Air Control (IAC)
Catalytic Converter
Muffler
Front Rotors
Front Brake Pads
Front Wheel Bearings & Seals
Oil & Filter x4
------------------------------------------

Leaking Coolant would not cause a severe misfire. If it did, it would occur at all engine temperatures and under any condition, not just under the ones I listed, and in addition the mis-fire would remain after having moved the truck.

Lets not get off track here

Re-read my original post and look at the conditions that lead to the misfire. I believe Will is spot on by saying it could be coolant temp sensor related.

Now, on that train of thought, if the motor is trying to start in "cold mode" and dumping too much fuel into the motor, what does an overly "rich" Air/fuel mixture smell like?

A vacuum leak WILL cause misfire, and it will be worse when the throttle plate is in the idle position.......!!!!

 

[rags]

I had the same problem with my 94 4.0 OHV. It was the lower intake! Upon disassembly I found the intake bolts loose, and after removal found that it was leaking coolent into the #3 intake port.

I was even getting alot of valve rattle ( pinging ) under acceleration.

My thought was when the engine was warming up the metal would expand and seal the leak.

After replacement mine runs better then it ever did! No more coolent lose and I don't use a drop of oil between changes.

 

[Twizzler09]

Brian, I know the intake could be causing problems... but as I recall thats upper intake? I think. Heres a pic(s) of the intake leak, and yes, this is the upper intake. I have what looks like water/coolant coming up from inbetween the intake chambers and the bolts.

Bob, I know vaccum leaks will cause misfire, thats proven fact

Let me try to re-explain.... theres essentially two stages of mis-fire.

Stage 1: Happens all the time, under any condition. An occasional miss, once every 2-7 seconds. No Mis-Fire codes.

Stage 2: Happens under the previously explained circumstances. An odor from the exhaust, somewhat strong and smells like a mixture of Coolant and over-abundance of gasoline. Revving the motor or going in reverse does not make it stop, as soon as I move forward even a foot, the mis-fire is gone and it reverts to normal Stage 1 missing. No Mis-Fire Codes.

Basically... the computer is under the impression that the motor is running perfectly fine the whole time this is happening. Which is why its making this exceedingly difficult to diagnose.

I really have trouble believing that an intake gasket could cause a *directional* problem.... and I cannot afford much "down time" as this is the only vehicle I have. I really wanna try and get this the first go-around.

In other words, someone explain why a gasket would cause a directional problem. I'd like to be fairly confident that it'll solve the problem before I do this

 

[Bob Ayers]

Brian, I know the intake could be causing problems... but as I recall thats upper intake? I think. Heres a pic(s) of the intake leak, and yes, this is the upper intake. I have what looks like water/coolant coming up from inbetween the intake chambers and the bolts.

Bob, I know vaccum leaks will cause misfire, thats proven fact

Let me try to re-explain.... theres essentially two stages of mis-fire.

Stage 1: Happens all the time, under any condition. An occasional miss, once every 2-7 seconds. No Mis-Fire codes.

Stage 2: Happens under the previously explained circumstances. An odor from the exhaust, somewhat strong and smells like a mixture of Coolant and over-abundance of gasoline. Revving the motor or going in reverse does not make it stop, as soon as I move forward even a foot, the mis-fire is gone and it reverts to normal Stage 1 missing. No Mis-Fire Codes.

Basically... the computer is under the impression that the motor is running perfectly fine the whole time this is happening. Which is why its making this exceedingly difficult to diagnose.

I really have trouble believing that an intake gasket could cause a *directional* problem.... and I cannot afford much "down time" as this is the only vehicle I have. I really wanna try and get this the first go-around.

In other words, someone explain why a gasket would cause a directional problem. I'd like to be fairly confident that it'll solve the problem before I do this

If you think you know it all, why are you asking for help?????

 

[shadetree]

It is obvious that you have a water leakage problem with the manifold. I would take care of that problem and not worry about what happens when you back up. Repairing the intake will probably take care of all your problems, real or imagined. The gaskets can be replaced in a few hours in a weekend. shady

 

[rags]

EXACTLY!

You know you have a water leakage problem! That is first and foremost the one to repair, ALL the other problems could and probably are the result of the obvious one.

It does not matter when it misses or how bad. coolent is getting in the combustion chambers as you have mentioned by the smell of coolent from the exhaust, and in short order will fubar your entire engine.

When that happens an engine replacement will be sure to solve everything.

 

[MAKG]

And FYI, burning coolant can poison oxygen sensors (it contains oxygen...), leading to a false lean condition. This might explain the fuel smell as well.