Rough hot idle & intermittent surges under light load

Hi all,

I have been experiencing a problem since I replaced my intake manifold a few weeks ago. I've been trying an array of different things, but so far no success.

I should also state that there are no MILs and no codes as read by two different scanners.

First I'll detail my problem
When idling in neutral or drive while the engine is cold, the engine idles very smooth. When idling in neutral while the engine is up to operating temperature, the idle is somewhat rougher and the RPMs jump up and down by about 80RPMs. While idling in drive, however, it is again very smooth. In addition to this, I noticed that when I start the car and the RPMs are higher, there is a noticeable misfire as heard in the exhaust and by a quick jolt of the engine. It happens maybe 5 or 6 times before the RPMs drop back to normal. If I open the throttle a little while idling in neutral, I can reproduce the same misfire.

The second part of my problem lies while driving around 35-45MPH (and higher, but not very noticable). In this speed range, with the engine at normal operating temperature, the TCC engaged, and under light load and throttle (no hills or accelerating), I will feel slight bucking. It feels as if I've hit a bump where there are none. It will do this up to three times per second when it happens the most and it happens intermittantly depending on the driving conditions.

Although I do not know if the two are interrelated, I did find a way to prevent the bucking, but only temporarily. If I unplug either the green vacuum tube attached to the EGR valve or the electrical connector to the EVR, the bucking does not occur while driving. The idle issue seems unaffected byt this. Since this indicated a DPFE-EGR issue with the bucking while driving, I've been trying everything I could to fix it.

Here is a list of everything I have done to try to fix these two issues:


spark plugs - replaced
COPS - cleaned and greased boots and springs
DPFE - replaced
vacuum hoses - tested for leaks/blockages
TPS - replaced
IAC - tested a different one
fuel injector o-rings - also inspected for vacuum leaks
fuel filter - replaced
STP fuel injector cleaner - no change
EGR valve - tested and cleaned
Replaced air filter
cleaned MAF
ran without MAF plugged in - no difference besides running richer

Incase it makes any sort of difference, I used the recommended Motorcraft parts for the TPS, spark plugs, and DPFE. Fuel filter is a Wix. Installed IAC is a Duralast aftermarket as the original had gone bad. I tested a Motorcraft one off of my dad's '99 Crown Victoria, however and made no difference (and his runs very smooth with half the mileage).

New coolant temp runs at 188-192*F, ATF at 162-175*F, and IAT indicates OAT +1*F.

I am contemplating changing all 8 COPs, but I am a little hesitant since the car performs well under acceleration with no indication of a misfire under those circumstances. I also am wondering if bad O2 sensors can cause this. All four are original with almost 97,000 miles on them.

If anyone has any tips or anything, it would be GREATLY appreciated. I have been trying to tackle it for over two weeks now with no success and want to make sure my car will last until 200,000+ miles.

You probably have a COP that is breaking down. If you know anyone that has Fors's IDS or PDS tool, it can pinpoint the bad COP. You can also check Mode $06 on a generic scan tool to see which cylinder is misfiring.

I used an OBD-II console in my laptop program to get the following. This was with the engine off (not sure if it has to be on):
Sent OBD-II SAE-J1979 message: Mode: $06 Pid: $50
Received OBD-II message: $41 $6B $10 $46 $50 $00 $00 $00 $07 $AE $78
Sent OBD-II SAE-J1979 message: Mode: $06 Pid: $53
Received OBD-II message: $41 $6B $10 $46 $53 $01 $00 $00 $00 $00 $26
Received OBD-II message: $41 $6B $10 $46 $53 $02 $00 $00 $00 $00 $98
Received OBD-II message: $41 $6B $10 $46 $53 $03 $00 $00 $00 $00 $F2
Received OBD-II message: $41 $6B $10 $46 $53 $04 $00 $00 $00 $00 $F9
Received OBD-II message: $41 $6B $10 $46 $53 $05 $00 $00 $00 $00 $93
Received OBD-II message: $41 $6B $10 $46 $53 $06 $00 $00 $00 $00 $2D
Received OBD-II message: $41 $6B $10 $46 $53 $07 $00 $00 $00 $00 $47
Received OBD-II message: $41 $6B $10 $46 $53 $08 $00 $00 $00 $00 $3B

I also did it later on with the engine running after a 100 mile drive in which I felt it again for sure on the highway. I had not saved the data unfortunately (battery died just before I got home), but I do know the PID $53 still showed the same values where the $00s. I suspect that indicates it is not detecting a misfire.

Do you know anyone that has the Ford scan tools? You can pinpoint the cylinder easily with it. Or you need an oscilliscope to check the coils. Hell, drive down here and I can find it for ya.:licka:

I don't know anyone with that kind of equipment. I am going to be taking the intake manifold off again in October during my fall break since I need to address a small leak coming from the gasket under the tstat housing. I'll replace both gaskets and maybe that will fix a small vacuum leak and solve the problem. Unlikely from what I've read though. Once I get the money I plan to get some new COPs too.