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Discussion Starter #1
My car develops bad hesitation and shuddering under acceleration after the car warms up, it gets worse and leads to bad idle at stoplights. Most of the engine sensors are new, along with a new fuel filter, and a new exhaust with new cats. Im pulling codes 21 and 74, though cleaning the connection on the ECT sensor made 21 go away. I'm wondering about my fuel pressure, but also about my thermostat. I'm going to change out my thermostat tomorrow because I'm thinking that it might be overheating and causing my fuel mixture to lean out and slightly vapor lock. The other alternative is a fuel pressure regulator, which is why I'm going to check my fuel pressure tomorrow. Any other ideas?

What else could be causing this problem? I am deathly afraid of melting my new cats.

There was a 95 3.8L Taurus wagon that came into my work a while ago that was throwing similar codes and having similar problem, ended up the computer was fried. Could that be a possibility with my car?
 

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Check for vacuum leaks.

What makes you think its overheating ?

PCM rarely goes bad, usually bad inputs from the sensors. New sensors dont always work properly. Monitor your A/F on the o2 sensor signal.

Oh yeah .. check for blown Head gasket also, do a compression and leakdown test.
 

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Discussion Starter #3
Check for vacuum leaks.

What makes you think its overheating ?

PCM rarely goes bad, usually bad inputs from the sensors. New sensors dont always work properly. Monitor your A/F on the o2 sensor signal.
My gauge doesn't work, or actually it barely works (only moves on the very low end of the spectrum) so I can't be totally sure its overheating but it feels hotter than it should be when I open the hood. As far as my sensors I think it might be the wiring going to them causing the issue. How do I know what the A/F mixture is from the o2 sensor signal? What is the range I should be looking for?
 

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My gauge doesn't work, or actually it barely works (only moves on the very low end of the spectrum) so I can't be totally sure its overheating but it feels hotter than it should be when I open the hood. As far as my sensors I think it might be the wiring going to them causing the issue. How do I know what the A/F mixture is from the o2 sensor signal? What is the range I should be looking for?
You wont know the AF mixture unless you smog it .. the o2 will tell you if its switching rich to lean but not adaptive fuel trims .. excessively lean or rich will usually trigger the MIL.

Do a continuity check on your wiring.

I doubt vapor lock .. thats oldschool. More likely blown HG.

OBD1 sucks.
 

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Discussion Starter #5
You wont know the AF mixture unless you smog it .. the o2 will tell you if its switching rich to lean but not adaptive fuel trims .. excessively lean or rich will usually trigger the MIL.

Do a continuity check on your wiring.

I doubt vapor lock .. thats oldschool. More likely blown HG.

OBD1 sucks.
It passed smog with flying colors (like 1s and 2s for the numbers of NO and CO2) like 500 miles ago, I was going to have the O2 sensors replaced but the old ones tested fine and my smog scores was low. Ive been told that diagnosing a lean/rich problem on an OBD1 Ford is one of the most difficult things as a mechanic. You think my head gasket could be blown between cylinders causing the problem? I'm not burning oil or coolant, and I don't have chocolate milkshake in my oil pan or my radiator...
 

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Your car runs rough .. could be a lot of things. Blown HG, EGR stuck open, vacuum leaks, bad plugs / wires, etc etc ..

Just because it passed smog 500 miles ago doesnt mean its not broken today.

I just fixed a 94 Mark, failed miserably on smog two weeks ago .. replaced the PCM and it passed today no problem.
 

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Discussion Starter #7
Your car runs rough .. could be a lot of things. Blown HG, EGR stuck open, vacuum leaks, bad plugs / wires, etc etc ..

Just because it passed smog 500 miles ago doesnt mean its not broken today.

I just fixed a 94 Mark, failed miserably on smog two weeks ago .. replaced the PCM and it passed today no problem.
New plugs, new wires, most engine sensors are new...

It could VERY possibly be the EGR considering the rest of the system got screwed up when my cats fried...
 

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Unplug the vac line from the EGR and see if it runs any different.

Ive seen new plugs fail within the first couple hundred miles .. just sayin.
 

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My gauge doesn't work, or actually it barely works (only moves on the very low end of the spectrum) so I can't be totally sure its overheating
There's a TSB on bad sending units for the gauge temp sender - all 1-wire senders to something like 1996 or so.

I'd swap that sending unit quick.

They're not that much - down to $2.44 at RockAuto. Add in a gallon of 50/50 (ideally, a gallon of concentrate and a gallon of distilled water, make 2 gallons of your own!), a small spool of PTFE tape, and you're good to go.

RwP
 

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Discussion Starter #10
Unplug the vac line from the EGR and see if it runs any different.

Ive seen new plugs fail within the first couple hundred miles .. just sayin.
I have another set of new plugs I can throw in there to see if it helps. Wouldn't unplugging the vac line to the EGR cause another vacuum leak?

There's a TSB on bad sending units for the gauge temp sender - all 1-wire senders to something like 1996 or so.

I'd swap that sending unit quick.

They're not that much - down to $2.44 at RockAuto. Add in a gallon of 50/50 (ideally, a gallon of concentrate and a gallon of distilled water, make 2 gallons of your own!), a small spool of PTFE tape, and you're good to go.

RwP
The sending unit is new as of January, and strangely enough I have a 2 wire sending unit. Its a red and a white wire IIRC.
 

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Well plug it up with something, obviously.

Like I mentioned before .. new parts dont always mean they work correctly. Check your wiring for continuity first.
 

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Discussion Starter #12
Well plug it up with something, obviously.

Like I mentioned before .. new parts dont always mean they work correctly. Check your wiring for continuity first.
Wouldn't the EGR still be open then, I don't really know that much about HOW the EGR works.

How would I test the continuity? Put a multimeter between the connector and the sending unit?
 

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Let me use my Psychic mechanic ability .. ( grabs a bucket of chicken bones, shake shake shake .. throws them onto the ground .. ) "yes, this does not look good"

You can cap off your egr with a soda can. Stuck open would usually cause it to run rough cold also ..

Test continuity with a multimeter, yes .. unplug both sides of the wireharness.

If it doesnt act up cold, you need to check everything hot while its displaying the symptoms.
 

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Discussion Starter #14 (Edited)
Let me use my Psychic mechanic ability .. ( grabs a bucket of chicken bones, shake shake shake .. throws them onto the ground .. ) "yes, this does not look good"

You can cap off your egr with a soda can. Stuck open would usually cause it to run rough cold also ..

Test continuity with a multimeter, yes .. unplug both sides of the wireharness.

If it doesnt act up cold, you need to check everything hot while its displaying the symptoms.
Just went out and started it up, it ran rough. It ran the same with the EGR plugged in as it did without the EGR plugged in and the vacuum line blocked, the vacuum line was sucking alot of air. What does that indicate?

Checking my other vacuum lines I couldn't find any immediate vacuum leaks, though I will try again tomorrow when the sun is up.
 

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The sending unit is new as of January, and strangely enough I have a 2 wire sending unit. Its a red and a white wire IIRC.
That's the ECM's sending unit.

There's also one for the gauge.

I'm talking about the gauge's sending unit.

RwP
 

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Discussion Starter #16 (Edited)
That's the ECM's sending unit.

There's also one for the gauge.

I'm talking about the gauge's sending unit.

RwP
No, my gauge sending unit has two wires. Let me put up a picture. Both sending units are new.

 

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Discussion Starter #17
Okay guys, just drove my car 100 miles, here's how it went.

Cruising is fine, normal even. Its only on acceleration that the misfire and stumbling occurs. The idle is rougher than normal. Also I'm sure its running really lean because I filled up in Woodland CA and it only burned 2.01 gallons of gas to get to Chico CA (91.1 miles from station to station). That equates to 45.55 MPG which I would normally think was really awesome however its just another indicator of how lean its running.
 

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There's always a possibility that the symptoms could be all related you said it running hotter than normal, hesitation when warm and, running lean right? Well a weak fuel pump would cause all of these symptoms, check your fuel pressure at the rail. A fuel pump is definitely a possibility if you are getting all of these symptoms.
 

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What brand of plugs did you put in?

If you got 40mpg then you have either faulty O2 or MAF. May be time for a fuel filter but on me those usually so signs of horrible mileage.
 

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Discussion Starter #20 (Edited)
There's always a possibility that the symptoms could be all related you said it running hotter than normal, hesitation when warm and, running lean right? Well a weak fuel pump would cause all of these symptoms, check your fuel pressure at the rail. A fuel pump is definitely a possibility if you are getting all of these symptoms.
Going to check it tomorrow. I just need to figure out this issue so my car can be somewhat reliable and I don't fry my brand new cats.

What brand of plugs did you put in?

If you got 40mpg then you have either faulty O2 or MAF. May be time for a fuel filter but on me those usually so signs of horrible mileage.
I just replaced the fuel filter like 2 days ago. It doesnt have an MAF but the MAP sensor is new. I put in Autolite plugs, going to check the fuel pressure. I was running iridium for a while but i switched them for the Autolite copper plugs.
 
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