fluctuating idle / misfire problem

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thehamm99

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Okay, here it is.

(Yes, I searched quite a bit, but the search function results in very broad findings...basically come up with the same posts no matter what word I search).

So here's the issue: I have an idle that fluctuates, from 650-1000 when engine really cold, from 750-900 or so when engine is warm. Also, it sounds and feels (upon acceleration, and at consistent speeds, highway speeds, etc.) like there is a slight miss, or maybe just is running slightly rough.

Here's what I've done:

Spark plugs: replaced, with ap 764
Boots: replaced with new
Coil packs: did not replace, but lifted each one from plug with engine running, and they seem to be good.

Fuel filter: replaced - didn't seem clogged anyway
PCV valve: replaced - seemed fine anyway
Air filter: replaced - didn't seem too dirty
MAF - cleaned thoroughly with appropriate spray and reattached
EGR Valve - cleaned with spray and reattached
IACV - tried to use one I grabbed at pick n pull, same issue persisted, so I cleaned the original with spray and replaced it as well as the gasket.

Did a seafoam treatment through pcv line. It smoked heavily for a couple minutes at high rev, but that was it.

I noticed if I sit in park and rev it to just over 4000 rpm, it sounds like a very minor backfire...which doesn't happen if I only rev to 2000 or 3000 rpm.

A couple months back it stalled out twice when pulling up to curbs at very low rpm...but that problem has not persisted. Maybe something I replaced/cleaned fixed that part of it.

*No smoke, *no rotten eggs smell, *no check engine light, *no heavy hesitation, *no loss of mpg, *no dying out, still performs well.

I'd say it feels like the engine is performing at about 80%, and sounds accordingly.

So I'm wondering? IACV, EGR valve solenoid, Throttle position sensor? Cat converter? I'm trying to avoid just throwing money at different parts unless I have to.

If I can get a hold of an obd II scanner, will it pull any codes without check engine light?

I'm sure I left something out, apologies in advance.

Any help is very greatly appreciated.

Thanks.
 
martz0r said:
How many miles? Why no plug wires?
Click to expand...

Probably a Gen 2 with COP.

Possibly a minor vacuum leak.

Check the IMRC's to see if they're stuck.

Get the code reader.
 
Last edited:
Sorry, Martz...knew I'd forget something.
It's a 97, no wires. 86k.

Thanks, DLF...I'll try those two things.

*Also forgot:
I cleaned out the throttle body...wasn't too dirty
I sprayed some vacuum lines I could get to with starting fluid, no change in rpm.

Thanks guys.
 
thehamm99 said:
I cleaned out the throttle body...wasn't too dirty
Click to expand...

That may have been a mistake, there's a coating on the TB that you're not supposed to remove. Usually there's a sticker on the TB that warns against cleaning it.

HPIM2517.jpg
 
DLF said:
That may have been a mistake, there's a coating on the TB that you're not supposed to remove. Usually there's a sticker on the TB that warns against cleaning it.
Click to expand...

They make TB cleaner made for the TB's that say don't clean. This stuff does not melt the clear coat on TB.
 
Yeah, I've read both sides of the TB cleaning debate.

I wasn't rough at all. I sprayed the stuff onto a sock and used it to wipe some gunk off the blades, not much else. And I didn't really scrub hard or anything. Hopefully it'll be fine. And as said, it didn't improve or worsen the condition I'm having.
 
Take it to Ford or a shop that has the ability to run a power balance test. This will ID what cylinder is causing the issue and you can continue trouble shooting from there. I have had lots of COPs that were on the way out but not bad enough to cause a code, and this is always how I identified them. Not saying for sure a COP is the issue but the test will narrow things down for you.

-Alan
 
The Ford diagnostic tool, or a Snap On Motus or equivalent. All and all about an $8000 scanner. On the Ford tool it will show graphs of each individual cylinder allowing you to see which one is not running as well. The dealerships use it alot now when people have complaints of rough idles and stubbles that don't trip a code. I had an F150 that had the exact symptoms the OP described and it turned out the be a bad COP that just wasn't bad enough to throw a code. Now when I have one that is running rough I just take it up there and have them run the test so I know where to look.

-Alan
 
I've never heard of that either...but I am in Sacramento. So maybe I can take it to Power Balance Pavilion. ---bud um pum, ching---

Thanks for the suggestion though, Alan...I'll definitely look into that. I'm assuming then this test should cost less than just replacing the COPs??

Is there no other way to test COPs?
 
sounds like a marginal COP to me. common ford thing unfortunately.

figuring out which one is hard without a CEL or a big computer like Alan mentioned.
 
Okay...

Since I am balling on a budget here...

If it is indeed a bad COP (just one, not multiple), couldn't I get away with buying one COP and testing it out in each cylinder in place of the COP that's currently in use?

Of course this could take a couple hours, but wouldn't it work?

wow, that sounds really cheap, but it is what it is.
 
My computer can test them, I only paid $1200 for it

There's a few things you can do, just trial and error to narrow down your problem as well

First off, remove the IAC and replace it with a NEW one. JY parts of that sort are not a good maintainance purchase

Remove the plugs and check the tips of each, they should have a slight white film for a normal burn. Take pics and label each according to it's cylinder-let us see what ya got
Check the gap, should be .054

You said you checked the coils and they seemed fine, how did you determine that?
 
Not saying your tester won't do a power balance because I don't know which one you have, but a coil test won't always show them. My buddy has a Motus that will run a balance test on most cars, but not on my Aviator. The Ford power balance shows the weak coil where as the Motus coil test didn't.

-Alan
 
Sapper,

To test the coils I pulled each one off the plug while running, and heard the engine drop, ping, whatever you'd call it, each time. I guess that was me testing them. haha.

So I should remove and check plugs even though they've only been driven 200 miles?

Assuming it's a yes, I'll have no problem doing that.

While I have you here, can you refresh my memory on the cylinder numbering? (i.e. which cylinder is known as which #...so I'll label correctly).

Thanks for the help.
 
That's basically how you Check a coil without a scanner yes, and yes pull the plugs and check them now that you know they're all fresh, and recheck your gap, if you accidentally dropped one into the chamber and it hit, then the electrode got bent slightly, it doesnt take much pressure to change their gap by a large margin.

The condition of the burn should be well evident after 200 miles

You also mentioned that your RPMs are
Between 700-1000, when you're sitting at idle, is it more like it's sitting at 1000, and then falls on it's face to 700, then picks back up? This would be evidence of a bad IAC

And yes you can rotate a brand new coil from cylinder to cylinder, it's your time ;)

As you increase the RPMs the misfire is more noticeable, means it's a misfire under load, which indicates a weak coil

Now I'm gonna go thumb through my scanners manual...since someone kidnapped the scanner

I hate when you buy some new cool electronic device, and you get a huge manual for it, and of course one on your first thoughts is "wow this thing does a lot of stuff", but nope, it does just a few things, in a bunch of different languages ...DOH
 
Sapper,

yes, the idle does seem to be sitting at 800, then drops to 600 (instead of the other way around). But...

I put on a brand new IACV and TPS yesterday. Did not solve the problem.

Coil is next. Waiting for shipment.

Any other suggestions?
 
No I did not. Somehow I forgot that, even though I've read it several times. Arrg.

Is it likely that that matters?
 
I would also check for vaccum leaks and like sap said check the spark plug gap.
 
Okay, I disconnected the battery overnight and tried that. Nothing changed.

I got one new coil pack and put it in cylinder 5 (driver's side, toward radiator).

I noticed at first the idle problem seemed gone...then it seemed there still, but less noticeable, but the car still seemed a tad bit sluggish, and while sitting in park and revving to 4000, I still got the dull thud of misfire or backfire (not sure what to call it).

* There was a small amount of oil on the plug boot in that cylinder

(Have not pulled the plugs yet, or looked at the other boots. Will do soon).

* I've read that the oil on the boot indicates a leaky valve cover gasket. I'm sure that's possible. But could that also be causing the jumpy idle? The apparent miss?

*If replacing one coil pack improved the condition, would replacing them all help? I guess I'm not sure about the coil packs...Are they either "good" or "bad" or can they weaken over time (meaning just replace them all)?

Thanks.
 
thehamm99 said:
* I've read that the oil on the boot indicates a leaky valve cover gasket. I'm sure that's possible. But could that also be causing the jumpy idle? The apparent miss?

*If replacing one coil pack improved the condition, would replacing them all help? I guess I'm not sure about the coil packs...Are they either "good" or "bad" or can they weaken over time (meaning just replace them all)?

Thanks.
Click to expand...

yes. the oil will degrade the rubber and the coil will short to the head, causing a miss.

coils can become marginal over time, working in some situations, but failing when they warm up or under certain situations. they are rarely "good" or "bad".
 
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