Strange Misfire - 4.6 Bosch

StangGT5

Well-known member
Feb 4, 2019
295
130
Atlanta, GA
I bought this 2000 4.6 HSE about a year ago. It already had green coolant and ran great. It wouldn't overheat if you wanted it to. 130K or so and I am pretty sure it has the original headgaskets. I did some brake and steering work to get it on the road, but life went by and it sat for about 6 months or so, running every now and then. I put like a half tank of 93 in it on a test drive months ago, and that was it.

Now I that am interested in it again, it has developed an intermittent misfire that seems to go away with throttle. No check engine light or codes.

Symptoms:
Almost as soon as the temp gauge gets off cold, it develops this slight, intermittent misfire at idle that breaks up an otherwise smooth idle. You can hear it slightly in the exhaust, it is sort of a hummmmm miss hummmm miss miss hummmm miss hummm. The tach needle wavers slightly when it misses. It gets more constant as it heats up, like a mean test drive with a lot of throttle, but never constant.
Once up to temp, at idle if I quickly give it say 1/4 or 1/2 throttle it has a faint misfire-like hesitation to it. If you get it just right it will definitely misfire for a second before gaining RPM. If you hold it above idle you can rev the piss out of it and it is very responsive.
If I power brake it when warm, it is mostly smooth and powerful but does infrequently break up. Maybe once every 20 seconds.
After driving for a while and cruising at constant speed, every once in a while I can feel a quick little miss, similar to an old EFI Ford small block when the ignition module on the side of the distributor begins to fail.
It does not leak or use coolant, nor does it overheat.
It doesn't seem to break up under medium or harsh acceleration, but it might just be unnoticeable.

I have tried:
New Bosch platinum plugs and Denso wires (no change)
Two cans of Royal Purple octane booster and filling up with 93 (seemed to make the missing less frequent, but upon a long test drive I don't think it helped)
Swapping in a known-good MAF from one of my other 4.6 trucks.
Checking for vacuum leaks (couldn't find any when plugging off the ports or spraying cleaner)
Checked intake manifold bolts. Found 2 or 3 could use a 1/4 or 1/2 turn, but all were decently tight. (I installed the plug wires without removing the manifold)
Replaced the IAC tubes.
Pulled the new plugs, all looked the same with orangeish residue.

Fuel pressure test: solid 50+ psi
Compression test:
#1 (160) #3 (150) # 5 (145) #7 (145)
#2 (~159) #4 (150) #6 (~139) #8 (150)

Thoughts?
Crankshaft position sensor? Usually that's an on or off situation.
Coil? I've never had one fail, even at much higher mileage.
Head gaskets? The heat making it worse makes me lean this way. However, the compression numbers seem okay, and it doesn't leak or use coolant. Test was done cold though. I've only seen BLOWN head gaskets on Rovers (smoke, constant miss, exhaust leak, loud, etc.), so a failure as subtle as this would new for me.
Worn camshaft? Engine is very quiet, and I don't think a worn cam would cause these symptoms. It makes decent 4.6 power when accelerating and easily climbs above 4000rpm in sport mode.
 

terryjm1

Well-known member
Jan 23, 2011
1,486
375
Regretfully, with everything else you have done, coils seem like the logical next step. With a mirror maybe look at the coils in low light (darker the better) and see if you can see current leaking. Not in a LR, but I have seen coils behave like this after warm up.

I have also heard of a bad vehicle speed sensor causing misfire issues.
 
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StangGT5

Well-known member
Feb 4, 2019
295
130
Atlanta, GA
Regretfully, with everything else you have done, coils seem like the logical next step. With a mirror maybe look at the coils in low light (darker the better) and see if you can see current leaking. Not in a LR, but I have seen coils behave like this after warm up.

I have also heard of a bad vehicle speed sensor causing misfire issues.

Unfortunately, you are probably right that the coil is the next stop. While watching the live data, I noticed that when sitting at idle in drive the computer starts counting misfires on CYL 2. Given that cylinder had the highest compression on the bank, I'm thinking ignition. I guess it could be a leaky or stuck injector, but the upstream O2 values fluctuate similarly. Downstream Bank A is ~.650 and Bank B is ~.800.

P38s have a little more room back there than D2s, but the coil still isn't coming out without the intake. If I pull the intake, I'm probably just going to do the head gaskets too at that point.
 

terryjm1

Well-known member
Jan 23, 2011
1,486
375
“If I pull the intake, I'm probably just going to do the head gaskets too at that point.“

For the little extra cost, it is well worth not having to take it apart again either because the problem was something else or preemptively catching a future problem and not have to take it apart later. I’d even consider replacing the injectors.
 
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Gordo

Well-known member
You can do it without removing much, I had to remove an sai pump line and the put the hood in service mode. Not that bad. I got original could packs for cheap on eBay. Hasn’t missed a beat since. 2000 4.6. Obviously plugs and wires are a good idea too.
 

StangGT5

Well-known member
Feb 4, 2019
295
130
Atlanta, GA
I started down the HG path, fought a losing battle with the passenger side EGR tube and ended up completely rebuilding the engine. I went with a new coil and rebuilt injectors for good measure. I found the rod and cam bearings weren't in great shape, so it was time anyway.
 
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