Got a miss that isn't a miss....I need help.

[Codycobra84]

Ok, truck started acting up the other day and now it's giving me a strange problem. Basicly it has what sounds like a miss but it's not a miss. I just recently put a new crank sensor in it so I know it's not bad. Anyways what it's doing is spitting and sputtering when it's under a load and reaks of gas. When I'm light on the pedal it will rev up but still has a little trouble doing so. My first thing I thought was the distributor was bad, then figured it was the cam sensor(aka: stator/etc...). I changed the stator in the distributor put everything back together and still had the same issues. The smell of gas wasn't as bad, but the miss was still there. I went down the engine pulling each plug wire and starting it up one plug at a time checking to see where the miss was comming from but it gave the same results with each cylinder...i.e. there was a miss that never went away on any of the trys. I can't figure out if it's a fuel problem, not enough spark, computer, who knows... The only thing that I've done before this happened was changed the oil, and it was like 3-4hrs after changing the oil. I'm stuck and am losing my mind with this thing, someone tell me its something easy please.

 

-Cody

[Geonovast]

How old's the O2 sensor? How's the CAT?

[Codycobra84]

How old's the O2 sensor? How's the CAT?

 

Not sure about the O2 sensor, the guy I got it from said it had been changed but didn't say how long ago. As far as the Cat goes it seems fine, doesn't seemed stoped up or anything, I've been thinking about cutting it off anyways but then I just add more work to the problem b/c I'll have to find/buy/weld in a section of pipe.

 

-Cody

[Codycobra84]

Ok, so I just looked into something, and was messing with the TPS to see if it was out of adjustment. Well what I discovered was that the "A" and "B" test were it should be 5 volts was only showing like 4.6ish and then when I tried to adjust it the voltage wouldn't change at all and it stayed on 4.6ish volts from 0-full throttle. I can't remember if it's supposed to stay the same or not. Now when I tested the "C" and "B" test the voltage would change some but I'm not sure what it should/shouldn't be at. I'm not sure if that would make it a bad sensor or not, so that's why I was asking.

 

-Cody

[Oizarod115]

if your truck is a -90 check out this website here...

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

and then bookmark it. :bowdown:

 

make sure the arm of the throttle body is moving the arm on the TPS too, it can be installed with the throttle body arm not moving the TPS arm.

[Wildman]

you said this started after you changed the oil :hmm:

 

It's possible that you knocked a vacuum hose off.

 

You know the propane torch trick??? run a unlit torch around the vacuum hoses and fittings, and if the engine races, thats where you vacuum leak is.

 

Also, find the vacuum line to the EGR valve, and disconnect it and plug the hose, see it that makes any difference on how the engine is running.

 

Post what year, engine and tranny you have, and mileage on the clock too.

[Codycobra84]

It's an '88 MJ 4.0 2wd AX-15 5spd. with 196,500miles.

 

Yeah strange enough the thing gets fresh oil and has a fit about it. I'll check the lines when I mess with it this afternoon, but I don't remember knocking any off.

 

-Cody

[maddzz1]

Did you switch to synthetic? It may have cleaned up a bunch of gunk and clogged the new filter, and don't use a fram filter.

[Wildman]

Ya never know, it don't take much to knock off a vacuum line.

 

Another thing, you didn't mention any thing about checking the fuel pressure. Do that first before jumping on any more parts. Also with almost 200K, check the fuel pressure regulator, that runs off vacuum, and if that fouls up, it would cause a "skip" in fuel pressure.

 

Have you changed the fuel filter in the pass??

 

http://www.lunghd.com/Tech_Articles/Fue ... ostics.htm

 

http://www.lunghd.com/Downloads_and_Lin ... manual.pdf

[Codycobra84]

As far as vac lines I didn't see any that had been knocked off, how ever I did a compression test on it and I don't like the results.

 

From front to back the pressures avgeraged : 105/120/125/125/120/120

 

All of these were done with all the plugs removed on a cold engine, I'm gonna do a leak down test and see if/where the air comes out of. But any/all ideas will be thought about.

 

-Cody

[Codycobra84]

Ok, did a leak down test on the number 1 cylinder and don't look to good. I put right at 100psi into it and had a steady flow of air coming out of the oil cap on the valve cover. So I'm guessing that would show that the rings have gone out on that piston? :dunno: I'm not really sure the only other thing that I could think would be that a valve seal went bad and a valve wasn't seating like it should but I guess that would've made the air come out of the intake/exhaust depending on the valve. I guess the next step would be pulling the valve cover......yay, lol....unless someone knows something that I don't know.

 

-Cody

[aemsee]

Definitly sounds like a ring issue. Redo the compression test but add a few squirts of oil to the number one cylinder through the spark plug hole. See if the compression jumps up to about what the others are. This would be a good indicator of rings.

[Eagle]

Before you rip the engine apart, give the magic rebuild in a bottle a try.

 

http://www.cpillinois.com/yale-automotive.htm

 

Third one down, the EOK engine reconditioning kit. JC Whitney used to sell just the pellets and I have used them on tired engines with good (not "spectacular," but definitely good) results.