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Old 03-20-2019, 06:16 PM   #1
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Runs Perfect, Except Dies On DownRev To Idle.

I have read through a lot of threads about egr, iacv, vac/ exh leak, etc., but my problem isn't 'exactly' the same as others.

My car seems to idle perfect, anytime it is idling, and LTFT and STFT stay at 0 at idle.

My issue is that the engine dies everytime I let it drop to idle on its own.

I can keep it from dieing if I stay in gear all the way down to idle speed before I push the clutch.

Everytime it dies while I'm still rolling, I can let go of the clutch while still in 5th or 4th and it gently restarts and idles seemingly perfect as I continue to come to a stop, as long as I'm going very slow.

If I clutch-restart the car while still above 30-40 mph, the car will idle 'seemingly' perfect for a few seconds, then just die.
I can repeat it still rolling over and over again.
Everyone else with this issue complained of poor idle when idling, but my idle seems smooth and as though all is well when idling.

If I start the car, it will idle smooth all day so long as you don't rev it up and let it drop on it's own.

Once stopped and still running, it will idle the whole time like nothing is wrong.

Other clues are:
-My LTFT hovers erratically from +7 to +10 and STFT at -3 to +4 when I'm at part throttle, crusing 55 in 4th gear

-Car does not have idle problem until warmed up.

-All vacuum hoses are new, in place, and look good all over.

-I already checked for exhaust leaks and only found two by the muffler. Fixed them, no change. No other exhaust leak found. I checked the manifold, all over every pipe, junction, everywhere, no leak. Test was done with a powerful blower sealed to the tail pipe. (Worked very well btw! )

-When crusing or wot, exhaust smells of nox fumes. Other cars don't like riding behind me because of it.

-Fuel filter changed for maintenance. No change. Air filter and pcv are still new.

-MAF seems to read perfect and is responsive to all throttle changes on Torque app's graph.

- There is no smoke from exhaust, no power loss, no hiccupping at all. In fact, it runs amazing.

It just won't grab and hold idle on its own.

Sorry for the long read.
Thanks for any ideas.

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Old 03-20-2019, 08:38 PM   #2
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What year?
What engine?
How many miles?
How long has been problem?
DTCs?
How long since last ignition service (plugs/wires/coil)?
How long since O2 sensors replaced.
How long since TB/MAF cleaned?

ST/LTFTs < +/-10 = normal

Doesnt like closed loop

Have you looked at live data to see that the O2 sensors are cycling normally in closed loop?
Verification of VSS signal to PCM?

This seems key:

Quote:
Originally Posted by cheap sleeper View Post
My issue is that the engine dies everytime I let it drop to idle on its own.

I can keep it from dieing if I stay in gear all the way down to idle speed before I push the clutch.

Everytime it dies while I'm still rolling, I can let go of the clutch while still in 5th or 4th and it gently restarts and idles seemingly perfect as I continue to come to a stop, as long as I'm going very slow.

If I clutch-restart the car while still above 30-40 mph, the car will idle 'seemingly' perfect for a few seconds, then just die.
I can repeat it still rolling over and over again.
Based on that I would suspect a possible signaling/continuity issue between the CPPS and the PCM. Have you verified the circuit?

Paul
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2005 ZX4 SE 2.0L Duratec 5-speed 191K miles
2002 Wagon SE 2.0L Zetec 5-speed 131K miles

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Old 03-20-2019, 08:47 PM   #3
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Quote:
Originally Posted by Paul1958 View Post
What year?
What engine?
How many miles?
How long has been problem?
DTCs?
How long since last ignition service (plugs/wires/coil)?
How long since O2 sensors replaced.
How long since TB/MAF cleaned?

ST/LTFTs < +/-10 = normal

Doesnt like closed loop

Have you looked at live data to see that the O2 sensors are cycling normally in closed loop?

Paul
Sorry.
I should have included all that.

01 zetec zx3
120k
Problem has been a couple of weeks now.
No codes.
Wires, coil, and plugs have ~10k and look great.
Valve cover is good and under 10k since replaced.
Pcv, air filer, vacuum lines, ff, pcv hoses all new.



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Old 03-20-2019, 08:57 PM   #4
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TB is clean.

MAF seems to read correctly according to the Torque app, but, hasn't been cleaned.
I'll do that tonight.

Upstream was replaced a year ago, and has always shown a weird trace, rather than a smooth sine wave. It has always looked weird on a graph in real time (I'll post pics).

Downstream is original and has been dead for 12 years with no real issue.

TPS reports responsively on the app.

Fuel pressure stays perfect.




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Old 03-20-2019, 08:59 PM   #5
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Iac and egr have 120k also.

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Old 03-20-2019, 09:32 PM   #6
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If high mileage may be from that alone. Engine drops down fast enough to then drag it down to die as an older engine does not make enough torque to stop it from dropping to zero. It can stabilize with a second to adjust like you show (standing idle all day long) but not having that and engine in freefall the inertia takes it right past that point.

Check all normal idle air to engine like PCV positively not blocked in any way and need clean TB butterfly and bore right where they touch and IAC. Past that on one of mine I had to once all that was guaranteed open the TB butterfly up a very small amount to let more air in than normal. Older engines need slightly more air due to dragging down in normal idle, again, they have lost a lot of idle rollover torque. What ring and valve seal do when they flee due to engine wear. A compression test may show something there too.

That adjustment is touchy, you can easily get out of IAC range and then PCM sends you the typical rpm too high thing or the idle begins to hang high. There IS a small amount of adjustment room there though, I went between 1/8-1/4 turn more open on the butterfly setscrew and worlds of difference doing it. Mine mainly died when a/c went on at idle. Freshly charged so it had a lot of drag. If ATX car then watch your engine braking at throttle dropped off on decel, the braking is greatly influenced by the butterfly setting and it will likely change a bit to be less. Which helps your issue too, rpm drops at a slower rate.

Mine which used to die all the time hasn't done it in forever. Of course other things like ignition need to be in line too, watch the plug wires, they go bad very easy at pulling them to work on plugs.

The issue is that the IAC tries to respond but they do not like lightning, it takes a fractional second to move, you don't have that at engine freefall. Another thing.....if you never change the butterfly to be open more on an older engine the IAC simply opens more itself but again, that takes time. If you artificially bump the butterfly open then the IAC no longer has to OPEN, rather it will look to CLOSE meaning you already have enough idle air to keep it running hopefully. You didn't have to wait for a response from IAC as you already had the air amount built in.
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Old 03-20-2019, 09:49 PM   #7
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Consider too a vacuum leak showing up at the much higher vacuum of decel ONLY.

I mention it because the zetec has a crap super thin gasket section at the idle bypass port at the joint between TB and intake.

I've had that thin section pull out sideways to then reseat back in place before and it gave me fits finding it. The entire TB does not sit right on the intake surface flat, rather the bolt holes in the corners have slight 'stands' molded into them to hold up the TB assembly roughly .010" or so high on the intake. That airspace is what allows the gasket to pull out of place, it is not hard pinched to not move at all. The gasket as well with age loses a whopping amount of the physical 'body' stiffness the material has normally that would stop that, it becomes extremely flexible. I could see by the coloration of the silicone there that the gasket right there had been sucked out in the wind at least part of the time, it was twisting sideways out of place. Kooky, man.

My problems there were not the same as here but with a little imagination I could see the gasket moving at decel only to leak to help kill engine then falling back into normal place to run right. Far-fetched but not impossible.
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Old 03-20-2019, 09:50 PM   #8
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Quote:
Originally Posted by amc49 View Post
If high mileage may be from that alone. Engine drops down fast enough to then drag it down to die as an older engine does not make enough torque to stop it from dropping to zero. It can stabilize with a second to adjust like you show (standing idle all day long) but not having that and engine in freefall the inertia takes it right past that point.

Check all normal idle air to engine like PCV positively not blocked in any way and need clean TB butterfly and bore right where they touch and IAC. Past that on one of mine I had to once all that was guaranteed open the TB butterfly up a very small amount to let more air in than normal. Older engines need slightly more air due to dragging down in normal idle, again, they have lost a lot of idle rollover torque. What ring and valve seal do when they flee due to engine wear. A compression test may show something there too.

That adjustment is touchy, you can easily get out of IAC range and then PCM sends you the typical rpm too high thing or the idle begins to hang high. There IS a small amount of adjustment room there though, I went between 1/8-1/4 turn more open on the butterfly setscrew and worlds of difference doing it. Mine mainly died when a/c went on at idle. Freshly charged so it had a lot of drag. If ATX car then watch your engine braking at throttle dropped off on decel, the braking is greatly influenced by the butterfly setting and it will likely change a bit to be less. Which helps your issue too, rpm drops at a slower rate.

Mine which used to die all the time hasn't done it in forever. Of course other things like ignition need to be in line too, watch the plug wires, they go bad very easy at pulling them to work on plugs.

The issue is that the IAC tries to respond but they do not like lightning, it takes a fractional second to move, you don't have that at engine freefall. Another thing.....if you never change the butterfly to be open more on an older engine the IAC simply opens more itself but again, that takes time. If you artificially bump the butterfly open then the IAC no longer has to OPEN, rather it will look to CLOSE meaning you already have enough idle air to keep it running hopefully. You didn't have to wait for a response from IAC as you already had the air amount built in.
Thank you for those tips!

I'm going to tinker around with the car more tonight since I'm off tomorrow.

I'll give that adjustment a try all by itself, just to see what kind of difference I can get out of it.



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Old 03-20-2019, 10:08 PM   #9
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Quote:
Originally Posted by amc49 View Post
Consider too a vacuum leak showing up at the much higher vacuum of decel ONLY.

I mention it because the zetec has a crap super thin gasket section at the idle bypass port at the joint between TB and intake.

I've had that thin section pull out sideways to then reseat back in place before and it gave me fits finding it. The entire TB does not sit right on the intake surface flat, rather the bolt holes in the corners have slight 'stands' molded into them to hold up the TB assembly roughly .010" or so high on the intake. That airspace is what allows the gasket to pull out of place, it is not hard pinched to not move at all. The gasket as well with age loses a whopping amount of the physical 'body' stiffness the material has normally that would stop that, it becomes extremely flexible. I could see by the coloration of the silicone there that the gasket right there had been sucked out in the wind at least part of the time, it was twisting sideways out of place. Kooky, man.

My problems there were not the same as here but with a little imagination I could see the gasket moving at decel only to leak to help kill engine then falling back into normal place to run right. Far-fetched but not impossible.
I have had valve cover gaskets do exactly that.

Good call.
I will check that as well.

I never think about that seal because we aren't supposed to disturb it or whatever, but, I did pull the TB off of my other focus and put it back on with seemingly no issues.

A couple of ideas:

Since my car dies if trying to idle whilst rolling over 30mph, yet, is fine if I'm only going 5mph, could the MAF be wrong even though it is seemingly tracing perfectly on the graph?
Maybe it's not wrong about airflow, but, just not heating up to it's proper temp, then, sending a false intake temp to the ecu?

Another thought I had that I plan to check tonight is the timing belt could have stretched a little too much.
It's at 120k after all.

I read on here about someone who had their belt stretch just enough to retard their exhaust cam a couple of degrees and cause my dieing issue.

Thanks again for the ideas!



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Old 03-20-2019, 10:09 PM   #10
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My question about the MAF is more about my fumey exhaust issue.

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