How to tell difference between 4.2 and 4.0 engines? - JeepForum.com
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post #1 of 24 Old 05-08-2019, 01:36 PM Thread Starter
Joncure
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How to tell difference between 4.2 and 4.0 engines?

I'm wondering if there are any clear ways to visually tell the difference between the 4.2 and the 4.0 engines. I'm trying to figure out which I have in my 85 CJ7. I'm about 97% sure its an engine from an 88 wrangler, since it takes an 88 model oil filter (found that out the hard way!) and valve cover gasket.

The valve cover doesn't say 4.2 or anything else on it like I've seen on others. I also know its not the original engine since the PO told me that the spare engine that came with it was the original.

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post #2 of 24 Old 05-08-2019, 02:12 PM Thread Starter
Joncure
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One difference I know is that the valve cover on my engine has a lot of bolts (not sure the # right now, but in the teens), and the oil filter is the 20mm metric thread type. I think those suggest the 4.0
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post #3 of 24 Old 05-08-2019, 02:19 PM
Fourtrail
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Casting number on the block is the best. On a machined flat on the right side of the block between #2 and #3 cylinders. You may also need the head casting number if it has been changed from stock.

80 CJ-5, 74 CJ-6, 56 CJ-5
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post #4 of 24 Old 05-08-2019, 03:07 PM
87MJTim
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If I am not mistaken, the 88 YJ still had a 4.2. The 4.0 wasnt available in the YJ until 91.

So, if the engine came from an 88, its a 4.2
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84 CJ7, 4.0 (93 XJ) AX15 (Formerly T5)
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post #5 of 24 Old 05-08-2019, 03:11 PM
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Quote:
Originally Posted by 87MJTim View Post
If I am not mistaken, the 88 YJ still had a 4.2. The 4.0 wasnt available in the YJ until 91.

So, if the engine came from an 88, its a 4.2
This.

The 4.2 (258) was made until 1990

Hoss

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post #6 of 24 Old 05-08-2019, 03:16 PM Thread Starter
Joncure
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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."
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post #7 of 24 Old 05-08-2019, 03:19 PM
LJameson
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I also think all 4.2s have a spot for a fuel pump, while the 4.0 never did.

1990 YJ
1976 CJ7
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post #8 of 24 Old 05-08-2019, 03:27 PM Thread Starter
Joncure
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Quote:
Originally Posted by LJameson View Post
I also think all 4.2s have a spot for a fuel pump, while the 4.0 never did.

So where would the fuel pump be located for the 4.0? My engine has it on the block in front of the distributer.
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post #9 of 24 Old 05-08-2019, 03:49 PM
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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."
Nope!

Wikipedia the YJ, 4.2 was used until 1990

Now then, if it was out of a Cherokee or Comanche, that's a different story. Those motors would have had like 15 bolts holding down the aluminum valve cover IIRC.

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post #10 of 24 Old 05-08-2019, 07:58 PM
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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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post #11 of 24 Old 05-08-2019, 08:05 PM
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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.

Tim

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post #12 of 24 Old 05-08-2019, 08:17 PM
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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
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post #13 of 24 Old 05-08-2019, 08:25 PM
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Does it have a carb
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post #14 of 24 Old 05-08-2019, 10:45 PM
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One will run and idle great and not leak, the other pukes oil and emits blue smoke. Lol. Good luck.

Look it's a 4.6!
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post #15 of 24 Old 05-09-2019, 09:28 AM
JoonHoss
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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

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