Mechanical Brain Tease


#1

As some of you know, I often maintain that problems can exist in spite of a no-DTC situation, multiple testing methods, etc. This problem occurred with my Lincoln a week ago Saturday. The car is a 1996 Lincoln Mark VIII with a 4.6 DOHC.

SYMPTOMS. Ran fine. Started 2 hours later and ran like garbage. My wife insisted it would clear up; I wasn’t convinced. Two miles of bucking later I pulled over, took the inexpensive code reader out of the console and it showed nothing. No CEL illuminated either. Two miles of bucking back home and we drove another car. On the trip that evening I picked up a new set of plugs as I knew I would be going into that area although the plugs (20k miles) and wires (30k) are comparatively new. Plugs are mildly difficult to access and upon initial removal of the plugs last Sunday I was 99%+ certain where the problem was after looking at the plug tips.

While in there I did a few other checks on a whim although some would have no bearing on this particular incident. Here’s the results.
No DTCs set and no CEL at any time; steady or flashing.
Compression - 180ish PSI all 8 cylinders.
Fuel pressure -35 PSI.
Fuel pressure regulator good and operative.
Manifold vacuum - 20" rock steady even with the poor running and no vacuum leaks.
All injectors ohm out at 14.7 and pulse.
Plug wire resistance as follows with short wires (15-16") in the top bunch and and long wires (24-26") in the lower.

  1. 6600 ohms.

  2. 6590 ohms.

  3. 6260 ohms.

  4. 7540 ohms.

  5. 9470 ohms.

  6. 11110 ohms.

  7. 9120 ohms.

  8. 9230 ohms.

There is one thing about the symptom I’ve omitted so as to cut down the odds of a lucky guess by anyone.

Just posting this as a minor mechanical brain exercise more than anything and the question would be; what is your line of thinking as to the cause of the problem and what would you do?


#2

Let’s see, you were 99% certain where the problem was after looking at the plug tips…
that would suggest that they were probably either fouled due to oil ash, fouled due to carbon deposits (which would suggest rich operation pointing to any sensor that feeds to the ECU algorhythm…VSS maybe? I’d expect a fault code with most of them), or there was evidence of heat damage (cracked center electrode insulator perhaps?)…

These are WAGs…can you offer any other hints?


#3

My guess: fouled plugs indicate PCV valve sucking oil into the engine.


#4

The plugs were not oil fouled and the engine was not running rich. One hint might be that all of the various engine management sensors and PCM are not the cause.


#5

Evidence was on plug tips.
Not fouled/rich
No vacuum leak
Therefore fuel starvation? Due to clogged injector?
Or steam cleaned from head gasket leak?


#6

Not fuel starvation, no clogged injectors, and it’s never been cleaned with anything.

Let me just add that the gasoline is clean and fuel quality is fine.


#7

I give up. Clearly beyond my amateur skills. :wink:


#8

The fish are nibbling. So my first guess is, some sort of intermittent restriction in the exhaust system. The next area I would suspect is the ignition system.


#9

White or yellow crystals on the plug tips would indicate coolant leaking into the combustion chamber. Blown head gasket?


#10

The thing that I notice is that you have two spark plug wires that have about 20% higher resistance than their companions. I am not sure what effect that has, though. Does that drop the voltage across the spark plug tip, and thus you might have a less powerful explosion or incomplete combustion in two cylinders? I feel like this would initiate the check engine light, though.


#11

Ford, no codes, replace fuel pump and clean and check wires and connections. My WAG.


#12

Since it is WAG and no codes, I say mice in the air intake/filter.


#13

Cougar and sestivers are heading in the right direction with the ignition. The use of the word “tips” is plural but the problem was singular and those 2 wires with slightly higher resistance was not the problem.

The car would actually run fine on the old plugs; it’s just that I had a new set and threw them as they’re a bit of aggravation to get to. I actually had the car running fine the next day.

My wife originally thought it was “bad gas” and I told her it felt like a specific cylinder miss.

Since this has gone in the ignition direction and is singular the only thing remaining is to guess which cylinder went belly-up. Guessing is a 1 in 8 chance… :slight_smile:

(Glad to see you around mountainbike; it’s been a while) :slight_smile:


#14

Is this one of those engines by Ford that used Reverse polarity? Where the plugs on one side of the engine was actually different then the other side??


#15

No; just normal twin coil packs and a single spark plug per cylinder on 4-valve heads.


#16

I’m going to say that the coil pack that fires cylinder 4 took a crap?

Tester


#17

I would have a look at a lot more scan data and do some ignition testing before even getting close to pulling spark plugs and wires, but you refer to being able to get on the right track by looking at a spark plug tip, and you say not oil or coolant fouled. I’m guessing you saw raw fuel on the plug, and found no spark output from one of your coil towers.


#18

But if there was no spark in one cylinder, why wasn’t there a misfire code?


#19

So something in the ignition area, huh. I suppose it wasn’t loose plugs or plug wires was it?


#20

The one spark plug wire over 11K would make me suspicious, but it sounds like that’s not the problem here from what you say. So w/these symptoms, no DTC’s, I’d suspect either a problem with unmetered air into the engine, a sticking IAC, or a spark problem. Since your testing shows it is associated w/one cylinder only, the first thing I’d do is remove/replace the spark plug wires one at a time. If every cyl is firing correctly, the change in idle/run quality would be the same no matter which is removed. If one of them has no effect, that would indicate the problematic cylinder.