Rough idle, but not misfiring...

[jdSHO94]

Hi All,

I am hoping to get some input on a mystery issue that I can not seem to solve- 1994 MTX with about 259K, as stated it has a rough idle but it is not misfiring... the engine just seems to "shake" noticeably through the body of the vehicle. Here's the odd thing- it only happens at idle (~800 RPM) and between ~1000 to 1100 RPM. Past 1100 everything smooths out as it does around 900 RPM as well. The best way to describe it is that it feels as though something is out of balance in the engine, but only at very low speeds.

The car drives well at all speeds above idle, perhaps with a little hesitation under 4000 RPM (it almost feels like the engine can't decide what to do with the spark timing). I have been prepping the vehicle for a cross country move and have done almost every conceivable bit of maintenance possible, including:

• front 60k (inc cam and crank sensors)
• valve cover gaskets
• spark plugs (motorcraft)
• plug wires (delco)
• IAC
• TPS
• O2 sensors
• intake to head gaskets (the metal ones)
• evap purge valve (I had the hose connected directly and it caused a huge vac. leak)

I have confirmed the engine is timed correctly and the crank bolt is tight (not crank cancer). I also replaced the clutch and flywheel recently (PP failure) and it had no effect on the vibration. I have been suspecting I may have a small vac. leak somewhere and I have been doing everything I can to track it down, but so far nothing. I have a vac gauge in the car, and I typically see 20-22 inches at idle depending on load... also no codes- the last time I checked codes, the only one I had was for that purge valve which was disconnected at the time. I have run into a wall as to where to go next, and I would greatly appreciate any input you may have.

 

[shomethe$$$]

Besides from in-proper firing order, wrongly gapped plug or high resistance in a wire. Try to put your hand in front of the MAF and see if it stalls, if it doesn't, you still have a leak. Or use some starting fluid to track the vacuum leak. Some people like CARB or brake cleaner. A loose clamp or inlet hose cracks are common on a SHO. If you got a new IAC, perhaps trying another one would be better. If you can blow through your IAC that means its stuck open or the plunger is not sitting right, with no power it should seal tight and not let air pass to the other port. Also disconnect the MAF and see if it smooths out (You may have to gas it to like 1500 to keep it running). You can try cleaning the MAF but I haven't noticed much difference with the sprays they offer. A dirty throttle body or obstructions would cause what your experiencing at least with the low idle. Notorious on Toyota Camry's and very hard to find since it never throws a light. 800 is low, 900 is correct. Also check for a good connection for the intake ground, battery to starter etc. Most of these checks won't cost you a dime.

You didn't specify the type of plugs, platinum or something else? Also high octane on a stock car makes it run rough on idle. Make sure you secondaries are not stuck open.

 

[jdSHO94]

Thank you. The firing order is correct (triple checked wire routing from coil to plugs) and the vehicle will stall if airflow is restricted to the MAF. Unplugging the IAC causes the idle to drop really low (~500 RPM) but it will not stall. I have been chasing the vacuum leak with throttle body spray and had no discernible results. Unplugging the MAF while the vehicle is running causes the engine to stumble and set the CEL, but it does not stall. The throttle body has also been cleaned recently, and the idle isn't really low, it may just be my perception of the tachometer- it idles at the same speed it always has, which I see as being about 800 but may be 900.

I will have a look at the ground wires again but everything looked pretty good at last check. The spark plugs are the Motorcraft fine wire platinum. This is already the second set of plug wires as the Denso ones had a wire with high resistance, and that one presented in a hesitation on acceleration (right off idle) and occasional misfire while driving... it doesn't misfire on me now but it could be incomplete combustion. Like I said it feels almost like something is out of balance. I appreciate the input.

 

[shomethe$$$]

Motorcraft fine wire platinum doesn't sound OEM but someone may know better, I've never ran anything put copper autolites, but mine was far from stock, so I would do some searching on that. I'm going to assume its not a low idle issue, its just a hesitation issue below 4000rpm. Check correction under the coil pack by taking it out. Have you driven or parked in heavy rain since the problem started?

About your IAC, is it new? Check for proper operation, on initial startup it should go up to about 1500 rpm before dropping down. While driving it should only drop to 1500 for a few seconds before dropping to 900 when coming out of gear.
This is where your needle should be:
http://www.ford-taurus.org/taurusinfo/G2/Cluster2.jpg

Your timing belt timing maybe correct but what is the base timing with a timing light? Someone can probably answer this better, but if I recall its 10 degree's or something. Sounds like the engine is not advanced enough. You may want to go deeper and check the crank and cam timing plates to make sure they are no chipped, corroded or bent. But those would usually cause a CEL. No real way to check if your cats are clogged, a real quiet idle is usually the tell tell sign.

Worse come to worse, the valve guides or valve seats are starting to go, a compression test may give you some direction. Or the cam timing chain tensioners or chain is stretched, hopefully you don't have to go this deep.

 

[shomethe$$$]

That should be corrosion under the coil pack. Also take out the main ECU connector and check of corrosion on the pin#1 and all pins both ECU side and plug side. Same goes for the CCRM.

 

[jdSHO94]

The motorcraft fine wire platinums are the AGSP32FM spark plugs, the OE replacemnt from the dealer that replaced the AGSP32PP's. The IAC is new, but I know that doesn't account for much with our IAC's. I might try ans acquire another one, it deinitely won't hurt. There is no water or oil in the spark plug wells, and thank you for the images of the clusters- using those images as a template, my car does indeed have a steady low idle- it sits just below where the needle in the image does on that gen 2. The hall effect rotors for the CID and CKP are fine, I was able to inspect them both with the recent serviice work. This issue was not immediate after doing that service work, i noticed it a couple of days prior to the clutch PP failing- if that makes any difference. The cats are recently replaced also, magnaflow y-pipe, and you would notice clogged cats more at higher engine speeds than idle. I doubt the valve guides are worn as the engine does not consume oil and as for the valve seats, it's possible but doubtful, as I would likely see the evidence of poor sealing (on the intake side at least) on the vacuum gauge. I will also inspect for corrosion under the coil pack and CCRM, as I know the ECU connector is clean (I have already checked for that issue as well). Thanks again.

 

[shomethe$$$]

No problem, I would check the resistance on the injectors as well long with a new fuel filter and pressure check throughout the rev range. I would check the TPS voltage, you do have some degree of mounting adjustment where it mounts to the TB, you'll have to search the specs. Same goes for the cam sensor, crank sensor gap. You've made a lot of parts changes.

If you had a tweecer RT you could check if the O2's are slow to respond. Honestly, you've changed way too much stuff to track down the issue or to keep up with maintenance, you may have to change or recheck each component again. Its scary to think that each component on your car is a different brand. You need to verify the timing and the other issues I discussed.

 

[jdSHO94]

Has anyone ever seen this kind of issue before? I would like to know if someone out there may have encountered this kind of problem (or something close to it) before and how it was resolved... through all of the searching I have done on the forum I haven't really seen much with concerns quite like this one.

 

[shomethe$$$]

I know you have a new clutch but this seems to be common

http://www.shoforum.com/showthread.php?t=125813

 

[jdSHO94]

Nope. Same symptom prior to clutch replacement.

 

[sperold]

I can't think of anything but plug wire routing. I know you have stated many times that it is correct, but does it look like this:
http://www.shoforum.com/showthread.php?t=89091

Are there no codes at all?

 

[jdSHO94]

Thank you, I just checked the wire routing again and confirmed that they are correct according to the link you posted. Could any of this be symptomatic of an ECU failing? When I worked for Toyota we saw some odd behavior from ECU's that were not working properly.

And that is correct, no codes are being set. I also removed each spark plug to see if perhaps an injector was not shutting off like it is supposed to, there was no evidence of fuel fouling on the plugs, no damage to the electrodes or ceramic, and the gap was correct.

 

[jdSHO94]

And it does indeed idle low when compared to the image linked to above-
that is the idle at operating temperature- is that something else I need to be concerned about?

 

[rubydist]

your idle response to unplugging the iac sounds perfect, as does your idle response to unplugging the mafs. your FM plugs are correct, and I never had any problem with them. I don't think you have any vacuum leak of consequence, based on your description.

I suspect you have one of two issues:
- a motor mount is either hard from age or soft from oil contamination such that the resonant frequency of the engine/trans assembly has changed to be right around 800 rpm (13 Hz). As soon as you speed up enough to get above that, all is well. Look over engine mounts carefully to see if one of them has a problem.
- a cylinder has low compression, just enough to not fire as strong at idle. Do you hear any piston slap/rattle when the engine is cold? You might consider doing a compression check.

 

[sperold]

This might be a place where that engine balance test that is performed in the OBDI test sequence comes into play. The theory is the test shuts down one cylinder at a time and notes the effect on the idle.
Try a scan with a code reader.