Stumble at idle

[Primer88]

When was the belt last changed?

It's original and has supposedly been checked twice and is still in good condition. I am going to get it replaced soon anyway though.

 

[Primer88]

I took it back to the dealership today. The PCM is bad. $830 to replace.

 

[rrbrian222]

The PCM is bad. $830 to replace.

Yikes. Time to high-tail it to the boneyard.......

 

[Sunk]

Damn, that's a lot. The shop I took mine to figured out the TPS and IACV aren't getting (either correct or any) input from the computer, so it sounds like it's the same deal as you.

It passed smog though, so it's not an urgent issue.

 

[Primer88]

I've done some more tests since yesterday that a member on another board suggested with no luck so far.

I started it and let it idle for about 10 minutes. The stumbling started immediately and was very regular.

• I checked for any arching (last night), there was none.
• I unplugged the MAF and drove it around in as many load conditions as I could for about 10 minutes last night and about 20 this morning. There was no change in the condition and there was no CEL. I ended that test when the truck would hardly run at all while I was driving.
• I checked the voltage on the red/green wire of each coil pack with the key on. The front one showed 12.15 and the rear showed 12.16, both were rock steady and neither of them had any change while wiggling the harness.
• I found that the driver's side spark plug on the second cylinder from the front was very loose (wire was not loose). I'm not sure how that got overlooked by myself and the Ford techs but it did. I tightened it and checked the others, no change in the condition. It seemed to have not made any difference with it loose or tight.
• I checked the 4 main grounds in the engine bay, they are all tight and only have the slightest amount of surface corrosion. They are not corroded on the contact points though. I also check all 4 ground points with my multimeter. They all read 13.6-8v with the engine running. I had the red lead of the multimeter hooked up the positive terminal on the battery and when I checked each ground I tried several spots on the connection. They all read the same voltage. I also checked about 6 random spots on the engine itself. Everything read 13.6-8v and there was no drop when the engine starting stumbling.
• I unplugged radio noise capacitor, no change in the condition.
• I checked the fuel pressure again. It still holds rock steady at 68psi while it idles fine and while it's having problems. I revved the engine up slowly and it stayed rock steady at 68psi even while stumbling.

 

[Primer88]

Replaced both coolant temp sensors and the TPS today because I was already working on my truck (ball joint replacment). No change in the condition at all. It still runs like hot diarrhea.

 

[Rackster]

A tough one!

Well, you've done quite a bit with the electrical/ignition systems, so assume that it is working as intended. Fuel pressure steady and you believe that the gas is good (I only trust the major providers by the way). Oxygen sensors new and other sensors appear to be working right. You have spark, oxygen and fuel. But are they all present at the right times.

Vacuum issues can make problems to. I assume you did a thorough review of that to. I notice my lines are looking their age, so I need to address them soon.

Did you happen to inspect the loose plug electrodes?? Any tell tale signs??

 

[Primer88]

I have been all over that engine checking for loose connections, vacuum problems and ground problems. I honestly can't find anything wrong. It's progressed to the point that it's nearly undrivable.

Since the problem started I've gone through at least 7 full tanks of gas. Pretty much all from different stations and brands (name brand and other). Around here I honestly don't trust "name brand" being "name brand". There are quite a few stations that were carrying the Shell or Chevron brand names on the signs and have mysteriously removed the brand and kept the color scheme. I can't remember the last time I actually saw a name brand tanker rolling down the road either. I grew up in southern california and the majority of tankers I saw before moving to the south nearly 5 years ago were branded with a major company, not "cooter's or billy bob's transport".

 

[Primer88]

Replacing the pcm didn't help. Well, there is a slight improvement in the condition but it's still horrible.

 

[Mirage]

Have you tried doing a compression test? With everything you've replaced and tried, it's the only thing I can think of that I haven't heard anything about. Perhaps you have a bad valve or piston ring leaking by.

 

[Primer88]

I have not done a compression test yet.

Here is a video I just took of what it's doing. The video was started after it had reached full operating temp and had just been driven for about 5 minutes. Click on the picture to view.



Sometimes it's better (wasted 10min of video when I first started it up, no problems) and sometimes it worse. Sometimes it will do what is being shown in the video (or worse) upon cold start up and sometimes it won't do it at all like when I tried to make the first video I mentioned in the previous sentence.

I have to say that since I changed the PCM this morning the stumble doesn't feel so "rough" as it did with the old PCM. The last few days I drove my truck before the PCM replacement felt HORRIBLE when it was idling funny and even worse when the issue was happening while driving. It felt like the bottom end was going to come out of the engine. Now it's back to a smoother stumble.

I will have a ScanGauge II tomorrow. Hopefully I will be able to extract some information with that.

 

[Rackster]

Fuel Suppliers

Yes, it gets tricky sometimes when a gas station that once was affiliated with one of the big providers goes private...but the changes are subtle and misleading. The same tactics have played out here in the N.E.. That said, my dad advised me long ago, pay the pennies a gallon more and stick to the major refiners. I only get unnamed fuel when the choices are that or walking.

Another thing to consider is hooking up with other folks and let them have a listen...talk to folks about what is going on. I had a 90 Nissan Stanza with the 2.4L that idled rough, which is a known characteristic of that motor of that era. However, mine ran rougher. I did everything I could think of to get it under control. Drove myself nuts. I sold the car and heard sometime later that the new owner did a plug change and the problem went away. That struck me odd as the plugs that were in were almost brand new. I conclude that one of the plugs was bad out of the box.

The point is that the new owner never knew about the fits I had trying to smooth out the engine. Without that 'distraction', he did what he thought he needed and it worked. Maybe a new set of eyes and ears will help you too. I feel bad that you are in a seemingly losing battle, but once you nail this, this community will benefit from it.

Kevin

 

[Primer88]

A new set of eyes is why I took it to the Ford dealer. They either kept my truck for days and didn't really do anything (which is totally possible) or they didn't find anything wrong with what had already been replaced. Unfortunately I moved here a couple months ago and I haven't really made any new friends yet so forums are my best bet as of now. Hopefully the ScanGauge will be able to tell me something. I am also going to see if I can rent a vacuum tester and give that a shot. If neither of those give me any results I will do a compression test. I'm holding off on the compression test as long as I can because the pain from my spine injury has been nearly unbearable the last few days and I'd rather not aggravate it if I don't have to.

A member on another forum has a truck with an identical powertrain (even uses the same PCM calibration code) and nearly identical truck overall. If anything, it's heavier than mine. He also lives in the same area as I just moved from but his city only gas mileage beats my best highway only mileage by a couple mpg and his highway only beats mine by a long shot. This is before I started having problems with my truck. Right now his his city only beats my combined by almost 10mpg. This has me puzzled as well. I know driving habits play a big role but since the 2.5L basically has no power I don't exactly hot rod it. The big difference in mileage between our trucks from the get go makes me wonder if there is another major underlying issue.

As far as the gas thing goes. There are actually very few "name brand" stations around here and the lack of "name brand" tankers still makes me skeptical about those stations anyway. Just about all of them are ancient as well and the apparent lack of regulation enforcement down here doesn't inspire confidence in any stations holding tank integrity or even pump accuracy. I filled up at a Chevron station when I first moved here that somehow pumped 18 gallons into my 16 gallon tank that wasn't empty, I was refunded for 10 gallons and never returned. I do not however base any gas purchases on price. They are all within a few cents of each other and more often than not the "name brand" stuff is on the cheaper end of the scale anyway.

 

[Primer88]

Here is what I've gathered so far from the scangauge.

No codes and no pending codes but it displayed "not ready" and according to the gauge's owner's manual that means "the vehicle hasn't completed some test cycles". I've put about 80 miles on the truck since I installed the replacement pcm.

As I was running the engine I had the following information displayed in real time:

• RPM
• TPS position
• Ignition Timing
• Water temp (irrevelent but none of the other options were giving me more important information)

RPM- the rpm's were obviously all over the place when the engine stumbled at idle, sometimes dropping to the mid 500's and back up just over 1000. RPM while idling fine was in the 775-810 range.

TPS- With the throttle closed it read "16", the number went up with no variations when the engine stumbled as I increased the throttle opening.

Ignition Timing- This is where things got funky. I don't know if the engine stumble at idle or at a higher constant rpm was the cause of the serious fluctuation in timing or if the timing fluctuation was the cause of the stumble.

At good idle it stayed primarily at 21 with a little fluctuation between 20-24
At bad idle is varied wildly from -8 to 20 but stayed primarily in the low teens
Good constant 1500rpm timing was 31 with slight variation +/- 1
Stumbling while trying to hold 1500rpm timing was again all over the place but not as bad as at idle and stuck around the mid 20s

Ran a vacuum test as well. It was hard to get a good reading for smooth idle because the engine was going ape**** the whole time I tried to test it. At what seemed to be a decent idle it stayed in the 16-20 range. When it got rough it stayed in the low teens. With the engine revved it stayed at about 18.

I made these 4 videos after I typed everything above. While they don't show everything I explained above exactly it's pretty close to what was going on in the tests I ran before the videos. Click on the picture to view the videos.

I did each test with the air conditioning on and off. Sometimes it doesn't matter whether it's on or off but this particular time having the a/c on made it much worse.

Vacuum test with the a/c off. It's idling fine until I rev the engine, then it starts having issues.


Vacuum test with the a/c on. Runs like crap pretty much the whole time.


ScanGauge with a/c off.


ScanGauge with a/c on.



Ideas?

 

[Primer88]

ok, it finally threw a pending code for misfire in cylinder #3. Hopefully tomorrow I will be able to do a compression test and check the plugs and wires for that cylinder.