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Discussion Starter · #1 ·
Hello! I have been struggling to figure my friend's 98 Mystique out, and brapple and I have tried numerous parts and remedies to help our friend Roger...

his Mystique has a 3.0L block with stock 2.5L heads, cams, and UIM/LIm/TB/ intake... which as someone else had posted somewhere, that the Compression ratio is up near 12:1 due to the stock 2.5L heads on a 3L block...

Here is what his message to me was, and he asked that i post it for him... he is a member of CEG (ctmystique)...


Ok here's my plight...

In the early spring I had spun a rod bearing in my 2.5L mystique so I did a 3.0 hybrid conversion with 2.5 heads and SE intake setup. Ever since then the cars has had a rough time starting. It starts ok when cold but recently after it gets hot it won't restart until it cools back down. I believe this may be due to high compression?? I tried replacing the starter but that didn't help the starting issue. It has also been idling very rough @ 750 rpm. If I bring the idle up to 1500 rpm it runs fine. I'm at my wits end trying to figure this out.

Here's what I've tried and or changed.

1. new plug wires (plugs were new at engine swap)
2. idle air control
3. cam sensor
4. crank sensor
5. engine wire harness(really bad insulation deterioation)
6. new starter
7. coil pack
8. maf sensor
9. SVT computer
10. SVT injectors
11. SVT exhaust

These are not in any particular order just as I remember that I've swapped.


I also am getting CEL codes

p301 misfire cyl 1
p302 misfire cyl 2
p303 misfire cyl 3
p171 system lean bank 1
p174 system lean bank 2
p1401 DPFE sensor circuit high voltage


I have a DPFE sensor that I'm going to swap out soon as well.

If anyone has had the same or similar symptoms I'd be eager to try some suggestions


It seems as if there isn't enough fuel being put into the motor while it idles, and it idles really rough!! Step on the gas a tiny bit, and it purrs smooth!

This car has not been tuned at all yet, but does have an SVT ECU and injectors... the car has been this way for a while and we keep trying different parts, some have helped a little, but not enough!!
It will start when the started is cool, and it shakes the whole motor when trying to start up, and then once it kicks on, it smoothes out for about 20 seconds, then hits idle speed of about 550-700, and it gets really rough with misfires and lean codes on both banks...

What is going on here? Why would this happen? and what parts could cause such problems?

We have been scratching our brains for weeks trying nearly everything, but something just isn't showing itself as the problem...

THANK YOU VERY MUCH FOR ANY INFORMATIVE HELP :)
 

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hve you checked the oval connectors on the passenger side of the engine bay. there is 3 of them. you should know which ones i mean. my guess is one of those connectors isnt sealing properly. i had it happen to me after my engine swap. now i have a zip tie holding it closed as i didnt know what else to do
 

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Hmm, as you stated, and by no means am I an expert, that the engine is NOT receiving enough fuel! This may be so stupid of a response, but have you guys checked out the fuel pump and/or fuel filter? I suppose it may be even a dumb thing like this that could cause this kind of situation...:shrug:
 

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Discussion Starter · #4 ·
Hmm, as you stated, and by no means am I an expert, that the engine is NOT receiving enough fuel! This may be so stupid of a response, but have you guys checked out the fuel pump and/or fuel filter? I suppose it may be even a dumb thing like this that could cause this kind of situation...:shrug:
I am not sure if Roger replaced that, but i am guessing that he did at one point since the swap...

And those 3 connectors are snapped in as tight as possible ... triple checked.
 

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some observations I made while doing some data logging on the car.

the IAC duty cycle drops to zero when the cars reaches idle. off idle the duty cycle is something other then zero

the maf seems to be reading correctly. I observed about 1.3# off idle, maybe around 2k rpm

off idle the spark advance is around 30 degrees as I have observed with my vehicles. however when this car reaches idle the spark advance drops to maybe 10 degrees where iirc mine always have stayed near 30 unless the coolant temp has reached 212 degrees and the fans are kicking on.
 

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I'm having the same rough idle problem right now as well, and I have codes for both insufficient and excessive EGR flow.

I just replaced my 2.5 about 2 months ago, but retained my SVT max-honed UIM. My old engine was burning so dirty (like, carbon all over the back of the car dirty), that I think the EGR channels in the UIM are probably clogged up, as well as the EGR itself. I tore it down yesterday, and I'm having the UIM, LIM, and TB hot-tanked to clean them out, then I am going to optimize the TB and LIM, and reassemble with new EGR, DPFE, and IAC. I'm also replacing the DPFE hoses, and blasting out the EGR tube to make sure that everything is cleaned out.

If this doesn't help my problem, I'm out of ideas for myself.


The no-start problem sounds very similar to what AZbobby is going through with his car, though it's a stock 2.5.
 

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you know...car not starting when warm, but fine when cold - sounds a lot like a typical OLD SKOOL problem - vapor lock. That is, the fuel line runs next to something HOT and when the car sits after shutting off, the gasoline in the line at that hot spot vaporizes.

Of course, I haven't heard of vapor lock on a fuel injected car, especially new ones, but it might be something to look into (where the fuel line runs and if it is next to any hot spots)? Maybe a fuel pressure regulator issue?

--george
 

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the intake and egr passages should all be clean. The lim was cleaned when engine was rebuilt and the UIM that is now on the car was powder coated so I believe its clean, either way its different from the original one so I think that rules it out.

I also was wondering if it could be an egr issue as I know if the valve if manually open at idle it causes rough running then the engine to die.
 
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