[8A_Honda]

So I'm at school (auto tech) and our instructor tells us about a car problem and wants to see who can figure it out by what he tells us. It's a 3.8L, has a random #1 cylinder misfire, compression is 150 on all cylinders except #1 is at 140 but leak down test says there's no leak in the cylinders. Apparently, this is a common problem on 3.8 motors.

I'm thinking it's the CPS because it can cause the misfire code and the timing would be off I suppose. But would that change the compression somehow??? It's just on #1 cylinder. Thanks would be appreciated.

[ESG-642T]

I would look at the valvetrain myself.

If it was the cps, I would suspect all cylinder to be reading a tad low...

Random misfire could just be a bad injector / ignition coil


Disclamer: I haven't eaten breakfast yet so only ESG is here the 642T is still waking up.

[Walters97Racing]

same idea here. remember the kiss method. keep it simple stupid. dont jump to the abstract possibilities first. check the plugs, wires, injectors and stuff that would have an effect on a single cylinder

[8A_Honda]

Yes but there's "lower" compression on cylinder #1. Leak down test says there's no leakage in the cylinder but it's 10 psi lower than the others. Random cylinder 1 misfire. Misfire isn't gonna cause the lower compression. Sure there's some information we don't have to answer the question but just trying to look like the smart one in the class. Lol. Well, I usually am.

[ESG-642T]

Originally Posted by 8A_Honda Yes but there's "lower" compression on cylinder #1. Leak down test says there's no leakage in the cylinder but it's 10 psi lower than the others. Random cylinder 1 misfire. Misfire isn't gonna cause the lower compression. Sure there's some information we don't have to answer the question but just trying to look like the smart one in the class. Lol. Well, I usually am.

valvetrain sir.

[8A_Honda]

Why do you think valvetrain???

[Walters97Racing]

well in that case.. is it the same piston as the rest? stupid question but its gotta be asked

[8A_Honda]

Yes it is.

[ESG-642T]

Originally Posted by 8A_Honda Why do you think valvetrain???

Low comp and leak down is fine, its my educated guess of today...

Without anymore specs on the motor of course.

[8A_Honda]

But there's a misfire involved. I would think that if this is a common problem, there'd be a tsb but we can't get onto alldata right now. Guess I'll post back sometime Monday if we don't get an answer before then.

[Walters97Racing]

lol. i got all data. want me to check it out? haha

[8A_Honda]

Sure, if ya have nothing else to do. Lol. I guess this could be the quiz for the forum as well.

[Walters97Racing]

lol. give me a min. i gotta log in n all

[Walters97Racing]

whats the vehicle?

[8A_Honda]

It's a Ford with a 3.8. He won't say anything else other than that it's around an 05+

[ESG-642T]

Originally Posted by 8A_Honda It's a Ford with a 3.8. He won't say anything else other than that it's around an 05+

05+ 3.8 , you mean 3.9?

Or its not push rod?

[Walters97Racing]

the only tbs i see remotely related is this

NOTE FOLLOW THIS TSB PROCEDURE ONLY IF THERE ARE NO SPECIFIC MISFIRE TSBs/SSMs RELEASED FOR THE VEHICLE SYMPTOM BEING EXPERIENCED.

ISSUE
Approximately 50% of coil on plug (COP) coils returned for warranty do not have a problem.

ACTION
The misfiring cylinder must be identified through Self-Test misfire codes or through WDS Power Balance. Rule out base engine problems, rule out fuel problems, and then look at ignition problems (be sure to rule out coil primary circuit issues). Once the above steps have been completed, and the issue is in the secondary part of the ignition system, the oscilloscope procedure outlined in this TSB can isolate the difference between a coil or spark plug problem.

[Walters97Racing]

yeah. the clossest thing i could find of corse for it to be a 3.8 was an 04 mustang

[nukeman13]

i had the same problem... but mine was bad o2 sensors and bad egr pressure feadback sensor... im sure if urs was the same then those codes for the sensors would come up... i suggest u listen to ESG.. he knows what he's talkin about haha

[Mr. Red Pony]

Possibly not a valve closing completely allowing just a little air to escape. That's my guess. But you would figure if it would let a little out, it would let all out. I think if it was the CPS it would cause it on all. Is the spark plug letting air past it? right before I blew out a sparkplug it was missfiring on that cylinder. Could be a combination of those WITH a faulty injector.. It is fuel injected right?