Runaway RPM

[jsark]

Over the last couple of days, my 98 SE (110k) has developed a runaway RPM problem. In short, it periodically wants to throttle up on its own. Here's what I've observed so far:

- It doesn't consistently occur. During any given duration of driving, it happens roughly 70% of the time.

- It only appears to occur after the engine has operated at highway speeds/RPMs for a while. For instance, at neighborhood speeds/RPMs, it hasn't occured.

- When it's apparent the engine is doing a runaway, I'll pop it into neutral to observe how far it'll go. It climbs steadily to between 3.5k and 4k RPM (never higher). Usually, in less than 10 seconds, it will drop off to idle RPM. When a runaway occurs, it always follows this pattern.

- Although it normally recovers from the runaway in less than 10 seconds, sometimes it's longer. A handful have lasted beyond 20 seconds.

- I can't tell precisely when a runaway starts, because I'm driving at a high RPM to begin with. Only when I want to coast or brake to a slower speed does it become apparent the engine wants to maintain the higher RPM. It's like trying to brake with your foot still on the accelerator.

My gut tells me this is a sensor issue of some sort. Can someone help narrow this down? I'm hoping it's a simple part swap...

 

[icysvt]

kinda sounds like a throtle hang. search for threads relating to that and see if any of it sounds similar tou your issuees.

 

[Heywood]

It could be the IAC, or I had this issue when my PCV valve was worn out.
I'd start with the PCV since its an easy thing to replace and <5 bucks. If you haven't replaced it yet its due anyway with that many miles.

 

[jsark]

Thanks for the replies, guys.

After searching/reading a number of posts regarding the suggestions you've made (which I'll certainly look into), I can't help but wonder if the TPS might be at fault.

Remember, the problem is not that the engine hangs, or idles high -- it wants to literally gun itself from less than 2k to between 3.5k and 4k. When it does finally release from the runaway, the RPMs drop immediately to idle (700ish).

 

[BrApple]

I would clean or repalce the IAC first. the tps doesn't do much for a mtx equipped car but it could be part of the issue.

and pending codes?

 

[jsark]

I've got a 98 SE (atx). Does that make the TPS more relevant?

 

[BrApple]

I've got a 98 SE (atx). Does that make the TPS more relevant?

from my understanding it most important in how the atx will shift.

 

[jsark]

Ah, I see. Well, I'll try the IAC first. See how that goes.

 

[BrApple]

Ah, I see. Well, I'll try the IAC first. See how that goes.

yeah try cleaning it and if it makes an improvement think about replacing it as cleaning it seems to only make it work alittle bit longer. make sure you go oem also, aftermarket ones are often trouble.

 

[Heywood]

Thanks for the replies, guys.

After searching/reading a number of posts regarding the suggestions you've made (which I'll certainly look into), I can't help but wonder if the TPS might be at fault.

Remember, the problem is not that the engine hangs, or idles high -- it wants to literally gun itself from less than 2k to between 3.5k and 4k. When it does finally release from the runaway, the RPMs drop immediately to idle (700ish).

As I mentioned before, I had this exact issue with a faulty pcv valve. Basically it wasn't closing, allowing extra airflow into the system and causing the revs to surge when I let off the gas. It would go up to 3-4k rpm and stay there for a while. Sometimes I'd have to rev it up and let off to get the revs to drop back down.

 

[BrApple]

my only thing with that is I have like 3 factory pcv valves and none seal completely. the only one that seals completely is one from a turbo coupe.


have you checked the throttle cable for fraying and sticking? would be another simple answer.

 

[jsark]

Understood, Heywood. I fully intend to pursue this as well, whether or not cleaning the IAC fixes the issue. The PCV is only $10 + shipping, so might as well.

For the sake of research/troubleshooting however, I'd like to try one fix at a time, from cheapest to most expensive. If anything, to benefit others who run across this problem as discover this thread.

 

[Heywood]

Understood, Heywood. I fully intend to pursue this as well, whether or not cleaning the IAC fixes the issue. The PCV is only $10 + shipping, so might as well.

For the sake of research/troubleshooting however, I'd like to try one fix at a time, from cheapest to most expensive. If anything, to benefit others who run across this problem as discover this thread.

That's the smart way to do it. Let us know how it goes.

 

[Dyoel182]

my only thing with that is I have like 3 factory pcv valves and none seal completely. the only one that seals completely is one from a turbo coupe.

Not many completely seal up anyway.

 

[ValCor]

My '98 CSVT does the exact same thing. It has been doing it for 10 years now. It comes and goes; but usually happens when I tend to get a bit frisky and run high revs. Usually it lasts for a few days, then may not happen again for months. It is a complete puzzle to me since it does not happen all the time and it never happens for my tech. I've just learned to live with it and chalked it up to Ford's great engineering. Sigh!

 

[Tony2005]

...For the sake of research/troubleshooting however, I'd like to try one fix at a time, from cheapest to most expensive....

That's the smart way to do it. ...

I can't help it but for some reason this makes me beam with pride.

Heywood mentions the issue with the PCV valve not closing. In my case, I had a broken hose and it stalled the engine during turns.

 

[jsark]

Update:

Cleaned the IAC yesterday evening. It was somewhat dirty inside, but definitely sparkled by the time I was done with it. Didn't bother test-driving it. I figured either it would fix the problem or not, which I could wait to find out on the drive in to work today.

Well, I discovered this morning that it won't hold an idle. Definitely wasn't expecting that. However, I drove it to work anyway, for troubleshooting purposes. I thought maybe it would regain the ability to idle once the engine was warm. To facilitate driving, I used my right foot to hold an idle (when necessary) and left foot to brake. Thankfully, 95% of my 22-mile commute is interstate.

The good news is, there was no occurrence of runaway RPM. The bad news is, it still wouldn't hold an idle by the time I arrived at work. The better news is, I think I found my problem.

Let me know if my logic is sound:

- Old, dirty IAC = holds an idle, but causes periodic runaways
- Old, clean IAC = won't hold an idle, but doesn't cause runaways (yet)
- New IAC = hopefully solves both problems

Anyone have additional ideas?

 

[Tony2005]

If you do plan to get a new IAC valve, get a Ford/Motorcraft one for the correct year. The aftermarket ones are hit or miss.

 

[jsark]

Yep. Planning on it. List is roughly $170. I've got a local stealership that'll do $125. Don't really wanna wait to have it shipped.

 

[jsark]

Well, bad news, unfortunately.

I replaced the IAC with a new part from my local stealership. Worked great for the first day and a half. Haven't had idle problems since, but the runaways came back. Don't understand why changing the IAC made the runaways temporarily disappear.

Anyway, I continued to drive it normally for several days for research purposes. It's basically back to square one. So, I decided to move on to the PCV. Picked one up locally (stealership $15) and installed it yesterday. Didn't change anything.

I've noticed the runaways are more frequent when the engine's been running longer and hotter. On the drive into work, I may get one runaway late in the commute. But in the afternoon, in 90-degree heat and thicker traffic, I get many runaways. I also noticed that running the A/C seems to increase the frequency, but I still get plenty of runaways after killing the A/C.

The runaways have a noticeable effect on ATX shifting. The ATX is slow to upshift at runaway RPMs (3.5k). Basically, it has trouble deciding to commit to the final shift (4th / overdrive?).

Something else that might help with the diagnosis:

Runaways only happen following application of the accelerator. Once the engine has calmed to an idle, it'll stay there until the accelerator is depressed and/or the ATX is shifted into drive. The most common runaway behavior is to accelerate to roughly 1.5k RPMs, take your foot off the gas to coast, but the engine wants to take you to 3.5k (if you slide it into neutral just to see what it does). Thankfully, this runaway behavior can be controlled by braking and the car's speed governing the RPMs somewhat.

I'm thinking of trying the hung throttle fix next, but at this point, I'm just shooting in the dark.

Sorry for the mini-novel. Any ideas?