Mongo

I took my 87 S4 for a ride down the freeway after work today and experienced some sort of surging. It was almost as if.the engine was skipping. I pulled into the slow lane and backed off to 2000 rpm. I applied gas again, and the car was fine! I then put my foot in it and did a kickdown to 2nd WOT run. No skipping or surging! Weird! It sounded like it had a slight misfire when I was sitting at a stop light prior to jumping on the freeway, but I had plenty of power up until the surging or skipping. Any thoughts on what this might be?

Mongo

On a side note, the car has a fairly new LH, wires have about 15,000 mi on them and plugs are a year old, but I noticed that there was the presence of oil in two plug holes prior to doing spark plugs last year, signaling the need to replace the seals. Could oil in the chambers may have caused the misfiring, or skipping?

Jadz928

That sounds to me like failing MAF symptoms. Are history of replacement?

Mongo

Hey Jadz, the MAF is new, well a rebuild from Roger back in 2009, Johb Speake stuff.

Mongo

Looking at my most recent smog notes, the tech said a lean misfire was audible. HC was high, and it barely passed.

Bill Ball

Even though the plug wires are "new", I'd be checking them based on my recent experience. It can be erratic.
https://rennlist.com/forums/9636915-post68.html

Mongo

Will do. Diving into it tomorrow before I make habanero sausages!

Mongo

I checked the connectors on the cap and coils. Both are clean as a whistle. Ran out of light and time last night but managed to run the car in the dark to look for sparks jumping under the hood. No sign of any. I still will change the plugs this weekend. I am driving it today and noticed a slight hesitation on take off from a full stop. Albeit I work only a half mile away, so I never really let it out much now.

Mongo

UPDATE:

I changed the plugs and found oil covering them in cylinders 1, 6, and 7. Looks like the spark plug seals are toast. I don't know if that's what could have caused the skip, but the plugs had a pretty normal burn on them with the exception of #3 (not covered in oil). That cylinder looked like it was running rich (leaky injector despite being new Ford ones?). After firing it up, it sounds like it runs smooth save for a slight uneven idle (still lean misfiring?)

To make sure my wires aren't screwed up, I ran the car in the dark and saw no sparks or fireflies under the hood. I still have a slight uneven idle though. The strange part is all the spark plug connectors, and the cap connectors are clean! The MAF is new and the car had the intake service. There are no vacuum lines leaking either. If it starts skipping again at cruise tomorrow between 2000 and 2500, I'm going to go nuts.

AO

When I read the thread, I immediately thought spark. Those plugs, 1,6, & 7 are all on the same coil/distributor. I would look a bit deeper. Could be a bad rotor, cracked cap. Check all things that are common among these plugs including the ground strap for the coil.

The one other thought is a loose ground on the engine harness back by the MAF. But that's more of a complete on/off symptom.

Mongo

Hey Andrew, I had a loose ground before on the LH that drove me nuts before back in Sep 2009. These symptoms are different, just like you said. I have confirmed the coil grounds are secure. The grounds themselves are still clean as well. I will have to check under the caps and rotors.

AO

The coil itself could also be going.

John Speake

Or the output amplifier for that coil.....

WallyP

Secondary ignition problems can be interesting...

The voltage developed in the secondary is determined almost completely by the voltage required to fire the plugs, or by the voltage required to get to ground some other way.

The voltage required to fire the plugs varies with throttle opening, since fuel/air mix is an insulator, and the more mixture you pack into the plug gap, the higher the voltage required.

It is not at all unusual to have a microscopic puncture in the plug wire insulation or the rotor, or a carbon track or crack in the rotor, distributor cap or coil top, that can allow voltage to leak to ground. When the engine is at small throttle openings, the required voltage is relatively low, so the plugs fire. When you open the throttle, more mixture is packed in the plug gap, and the required voltage goes up quite a bit. The increase in voltage can reach a level that allows the voltage to pass thru the puncture/crack/track to ground, so the plug doesn't fire.

If the misfire is one plug, the problem is usually on one plug wire or a crack/track in the cap.. If it is more cutting out, the problem is usually coil, coil wire, or rotor.

Bottom line - looking for fireflies at idle doesn't tell you much. The misfiring usually happens first near torque peak at medium to WFO throttle.

dcrasta

These plug wires are very durable. That said did you do cap and rotors at the same time? I know they are expensive but If they have 50-60k miles on them It may be a good idea. If not just inspect and wipe with emery cloth until shiny.