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Thread: How to tell difference between 4.2 and 4.0 engines? Reply to Thread
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  Topic Review (Newest First)
05-17-2019 07:13 PM
ShadowsPapa My 87 Comanche Sportruck had the 4.0 - also had uneven idle issues the dealership said "welcome to renix"...........

By the way - pictures

Black engine - 4.0 or 242, blue engine - 258 or 4.2
05-16-2019 03:51 AM
zouaveherb
Quote:
Originally Posted by Joncure View Post
Disclaimer: Now I'm quoting here from Wikipedia but...….

"The 4.0 engine, which was introduced in 1986 for the 1987 model year, was a further evolution of AMC's 258 six.[16] It had a 3.875 in (98.4 mm) bore and a 3.414 in (86.7 mm) stroke giving it a displacement of 241.6 cu in (4.0 L).[17] Connecting rod length was 6.125" - similar to the discontinued 199 which was phased out in 1970."
It was introduced in 1986 for the 1987 model year on the Cherokee/XJ. It was not used on the YJ until 1990 for the 1991 model year. Also, the YJ 4.2 had a 2-barrel Carter BBD carburetor while the 4.0 is fuel injected. I know this because I had a 1987 YJ and a 1990 YJ. My friend's sister had a 1991 YJ.

/herb
05-09-2019 01:22 PM
Joncure 4.2!
05-09-2019 01:22 PM
Joncure And manifold date stamp says '88
05-09-2019 01:17 PM
Joncure I'd say good work detectives! Yes my intake and exhaust manifolds are in line. It is carb'd (Weber 38), and the fuel pump location!
05-09-2019 12:35 PM
CSP
Quote:
Originally Posted by 87MJTim View Post
The OP indicated his fuel pump is on the block, in front of the distributor. If it is still functioning, then I do not believe he could run FI on the lower pressure the mechanical pump generates.

Conclusion: his engine has a carb.
Better yet, he has a 4.2. The 4.0 casting didn't allow for mounting of a mechanical fuel pump (which was already stated in this thread).
05-09-2019 11:08 AM
87MJTim The OP indicated his fuel pump is on the block, in front of the distributor. If it is still functioning, then I do not believe he could run FI on the lower pressure the mechanical pump generates.

Conclusion: his engine has a carb.
05-09-2019 11:01 AM
StrippedTorx
Quote:
Originally Posted by JoonHoss View Post
Quote:
Originally Posted by StrippedTorx View Post
Does it have a carb
A 4.0 can have/ run a carb too. They didn't come that way, but can relatively (and easily) be converted to run a carb

Hoss
I agree but that would be out of the ordinary. He has a 7. Somebody swapped in a 258 from a yj. If it was fi I would assume he has a 4.0. Not to many people around here running a 258 with fi.
05-09-2019 09:34 AM
KLG79 On a 4.2 the intake and exhaust ports are on the same plane. Inline.

On a 4.0 the intake ports are higher than the exhaust ports.


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05-09-2019 09:28 AM
JoonHoss
Quote:
Originally Posted by StrippedTorx View Post
Does it have a carb
A 4.0 can have/ run a carb too. They didn't come that way, but can relatively (and easily) be converted to run a carb

Hoss
05-08-2019 10:45 PM
RARECJ8 One will run and idle great and not leak, the other pukes oil and emits blue smoke. Lol. Good luck.
05-08-2019 08:25 PM
StrippedTorx Does it have a carb
05-08-2019 08:17 PM
BagusJeep if you look on the inlet manifold near the front you should find a round date stamp, this was the casting date of the manifold.

You should also find on the engine block the casting number.

AMC
1981-85
258 4.2 3.75 L-6 7/16" Head Bolts
3235444
3933000
3224480
3241656
3235477

AMC
1986
258 4.2 3.75 L-6 1/2" Head Bolts
3235444
3933002
3235477

AMC
1987-89
258 4.2 3.75 L-6 2.06" Crank Neck, Spec. if Alum. or Plastic Valve Cover, 1/2" Head Bolts
3235444
3661
3727
05-08-2019 08:05 PM
87MJTim
Quote:
Originally Posted by Joncure View Post
So where would the fuel pump be located for the 4.0? My engine has it on the block in front of the distributer.
The fuel pump on the 4.0 is in the gas tank. The fuel injected engine requires much high pressure than the carb engine. The FI pump pushes the fuel to the injectors; the carb engine sucks the fuel like through a straw.
05-08-2019 07:58 PM
CSP
Quote:
Originally Posted by Joncure View Post
Disclaimer: Now I'm quoting here from Wikipedia but...….

"The 4.0 engine, which was introduced in 1986 for the 1987 model year,"
The missing part of that statement relevant to the OP, is that the 4.0 was only found in the Cherokee and Comanche until 1991 when it made its debut in the YJ.
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