4.0L, gas in oil?

[cocco78]

Its not my DD, but I was driving it on 100 mile round trips twice a week to school to save wear and tear on my powerstroke.

[pingpong]

How many miles are on the motor... has the motor been abused. I built a Pontiac motor back in the late 90's and it had the problem with fuel in the oil.. mostly cause I had way to big of a carb on it.. as well as the rings not being seated properly after a rebuild. Changed carb... new plugs.. gas smell gone.

 

 

Things to check...

 

Compression

Then do a compresion leak down test (this will tell you where you are losing pressure.)

THen if you have an acceptable leakdown test Check your ignition components. Make sure each cylinder is firing

 

If all of that stuff is acceptable.. its time to go over the motor with a fine tooth comb.. I mean check all your sensors, all your vacum tubing, and all that other junk.

 

 

My money is on bad cylinder rings letting the gas leak by once the motor gets hot.. thus increasing tolerances

[FxRacing282]

well when metal heats up it expands thus making a better seal with the rings..

[JeepcoMJ]

yea, not when the rings are bad. they still expand, but they leave gaps all over the place.

 

..and eventually score the piss outta the cylinder if you let them.

[FxRacing282]

ahh right.. knew i was forgetting something

[87manche]

check the fuel pressure?

 

I wonder if the FPR is hosed, or the return line is blocked, forcing the fuel into the motor rather than back to the tank.

[cocco78]

I don't think the truck was abused at all before we got it, it was a 1 owner and the guy drove it back and forth 100 miles a day for a while... The truck has about 230k miles on it, the PO said the motor was rebuilt a few years back. Anyway, i'll spend a couple hours on it this weekend and let you guys know what I find.

[cocco78]

Finally did a compression test, had to order the tester.

 

#1 100lbs

2 110

3 110

4 100

5 115

6 100

[89MJ15]

Bumping this up 

My Truck has run maybe a couple times a year the last 5 years. Checking it out this spring and the dip stick was 4” high and smelled like gas 

[RustyRodder]

Dumb question time with background- on mustang cobras, its a known issue that the fuel rail pressure sensor diagphram can blow out, allowing fuel into the oil. Is there a route that something similar could happen with the regulator on the 4.0s? 

[Eagle]

On 4/29/2026 at 11:51 AM, RustyRodder said:

Dumb question time with background- on mustang cobras, its a known issue that the fuel rail pressure sensor diagphram can blow out, allowing fuel into the oil. Is there a route that something similar could happen with the regulator on the 4.0s? 

 

No.

[Pete M]

On 4/29/2026 at 11:05 AM, 89MJ15 said:

Bumping this up 

My Truck has run maybe a couple times a year the last 5 years. Checking it out this spring and the dip stick was 4” high and smelled like gas 

 

sounds like an injector is stuck open?