This is topic 91 Foxbody Misfire in forum Tech Talk at Northern California Ford Owners  .


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Posted by Rigo5.0 (Member # 5247) on :
 
- Car starts right away when cold, but runs with what seems to be a bad misfire
- Once warm, car will shut off and not start until engine cools down

I've checked/replaced the following:
- Spark plugs
- Coil
- TFI module
- Vacuum hoses

Anything else I should strongly consider checking?

Thanks.

[ 2015-04-28, 10:11 PM: Message edited by: Rigo5.0 ]
 
Posted by LXjames (Member # 10791) on :
 
Next time it does that, pull the spout connector and try to start it. If it starts you may have a bad pip in the dizzy
 
Posted by Rigo5.0 (Member # 5247) on :
 
quote:
Originally posted by LXjames:
Next time it does that, pull the spout connector and try to start it. If it starts you may have a bad pip in the dizzy
I'll give that a shot. Anything else?
 
Posted by LXjames (Member # 10791) on :
 
quote:
Originally posted by Rigo5.0:
quote:
Originally posted by LXjames:
Next time it does that, pull the spout connector and try to start it. If it starts you may have a bad pip in the dizzy
I'll give that a shot. Anything else?
What's your plug gap at at and what wires you running
 
Posted by NK2186 (Member # 12319) on :
 
Keep pulling plug wires until you find a cylinder that does not change when you unhook the sparkplug wire for that specific cylinder.

Check the plug in that cylinder and see if it is white or black.

Once you find the cylinder you can try swapping the fuel injector to a different bank and seeing if the problem follows. If it does then you just need to replace the injector.
 
Posted by Fostang (Member # 3752) on :
 
Have you checked the fuel pressure with a gauge?
 
Posted by dohcfox (Member # 12829) on :
 
Sounds like plugged cats. Loosen the exhaust system at the headers and see if it goes away.
 
Posted by LXjames (Member # 10791) on :
 
quote:
Originally posted by dohcfox:
Sounds like plugged cats. Loosen the exhaust system at the headers and see if it goes away.
If it was plugged cats it would progressively get worse with time starting with poor driveability then no start. It wouldn't get bad as it warms then go away as it cools. Plugging is permanent a temporary obstruction wouldn't be permanent but then again its theoretically impossible to temporarily obstruct an exhaust from the engine
 
Posted by dohcfox (Member # 12829) on :
 
quote:
Originally posted by LXjames:
quote:
Originally posted by dohcfox:
Sounds like plugged cats. Loosen the exhaust system at the headers and see if it goes away.
If it was plugged cats it would progressively get worse with time starting with poor driveability then no start. It wouldn't get bad as it warms then go away as it cools. Plugging is permanent a temporary obstruction wouldn't be permanent but then again its theoretically impossible to temporarily obstruct an exhaust from the engine
Opposite. As things heat up they expand. And the cats plug with carbon and excessive fuel. So when they are cold it allows small pinhole size amounts of exhaust per catalalist by. Then when it warms up they close up more and more. Furthermore the restriction backs back up into the intake manifold and the computer recognizes the hard to keep running condition, misfires read excessive o2 in the exhaust so it drives the fuel mixture richer and richer making the situation even worse. Then when you let the cats cool down the exhaust can make it back by.
 
Posted by LXjames (Member # 10791) on :
 
quote:
Originally posted by dohcfox:
quote:
Originally posted by LXjames:
quote:
Originally posted by dohcfox:
Sounds like plugged cats. Loosen the exhaust system at the headers and see if it goes away.
If it was plugged cats it would progressively get worse with time starting with poor driveability then no start. It wouldn't get bad as it warms then go away as it cools. Plugging is permanent a temporary obstruction wouldn't be permanent but then again its theoretically impossible to temporarily obstruct an exhaust from the engine
Opposite. As things heat up they expand. And the cats plug with carbon and excessive fuel. So when they are cold it allows small pinhole size amounts of exhaust per catalalist by. Then when it warms up they close up more and more. Furthermore the restriction backs back up into the intake manifold and the computer recognizes the hard to keep running condition, misfires read excessive o2 in the exhaust so it drives the fuel mixture richer and richer making the situation even worse. Then when you let the cats cool down the exhaust can make it back by.
That's probably possible but I've never heard someone have that problem. But just because it's rare doesn't mean it can't happen I guess. But in my thinking, if the ecu is sending a rich signal shouldn't there be an abundance of air and not fuel? I don't think the O2 would get an air mixture as it plugs thus causing the computer to go rich? Also if it is getting richer as it warms wouldn't that just eventually plug the cat all together instead of leaving pin holes like you said?

Edit: if I'm reading his signature right I'm guessing his 91 has a 331/t56 set up I would doubt he's wasting time with cats

[ 2015-04-30, 06:47 AM: Message edited by: LXjames ]
 
Posted by dohcfox (Member # 12829) on :
 
Didn't read the signature. Just going of what I deal with every day. I am a triple master ford technician that specializes in gas engine performance and electrical. So I see it once a week with 3v f150s
 
Posted by dohcfox (Member # 12829) on :
 
Could be anything really, but the common cause of a misfire and a no start when hot is pluged exhaust. Plus it's free to check for a pluged exhaust. Tfi issues or stator issues start and run normally then quit. Or are a no start, temprature has little bearing on tfi issues. And fuel delivery or pressure issues are more pronounced cold since the engine burns the highest volume of fuel cold.
 
Posted by Rigo5.0 (Member # 5247) on :
 
-Car does not have cats
-Plugs gapped at .054 (have been for the last year it's no issues)
-I will check the wires

Thanks guys. Anything else?
 
Posted by Rigo5.0 (Member # 5247) on :
 
quote:
Originally posted by Fostang:
Have you checked the fuel pressure with a gauge?
Yes. FP is at 39psi

[ 2015-04-30, 08:39 PM: Message edited by: Rigo5.0 ]
 
Posted by Rigo5.0 (Member # 5247) on :
 
quote:
Originally posted by NK2186:
Keep pulling plug wires until you find a cylinder that does not change when you unhook the sparkplug wire for that specific cylinder.

Check the plug in that cylinder and see if it is white or black.

Once you find the cylinder you can try swapping the fuel injector to a different bank and seeing if the problem follows. If it does then you just need to replace the injector.
I will give it a shot.
 
Posted by dohcfox (Member # 12829) on :
 
Use a noid light and see if the injectors are still being fired when there is a no start situation, +1 on the over fuel situation then on one or more cylinders. If it is being over fueled you should be able to hold the throttle at 3/4 open when cranking and get it to clear and fire.
 




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