• Hello Guest, we are proud to now have our Wiki online that is completely compiled and written by our members. Feel free to browse our Jeep-CJ Wiki or click on any orange keyword when looking at posts in the forum.

Loss in power on 4.2L

Loss in power on 4.2L
A muffler can plug up the exhaust also and kill power.
Did that 'shop' run a vacuum ck on the engine looking for a plugged exhaust. If not-Tell'm to do so.
What did they set the timing to?
What size tires are you run'n?
BTW-IT'S CALL'D A JEEP :pissed: :poke:
LG
 
Lumpy Grits said:
A muffler can plug up the exhaust also and kill power.
Did that 'shop' run a vacuum ck on the engine looking for a plugged exhaust. If not-Tell'm to do so.
What did they set the timing to?
What size tires are you run'n?
BTW-IT'S CALL'D A JEEP :pissed: :poke:
LG
Click to expand...
They didnt do a vacuum check. I am also going to go ahead and order a flow master muffler and get that installed. I think I am also going to just order a rebuild kit I found a complete new kit on Ebay for 380 bucks hopefully that will resolve the issue since I will be able to tear apart the engine and see whats really going on with the Jeep.
 
WHY the rebuild kit?:confused: :eek:
Who's is sell'n this 'kit'?
A vacuum gauge is cheaper than a flowmaster.
Stop throw'n $$$ at it and fix'n what ain't broke.
AGAIN I'll ask, what size tires are you run'n on this jeep? What's your ring & pinion ratio?
What tranny are you using?
LG
 
Lumpy Grits said:
WHY the rebuild kit?:confused: :eek:
Who's is sell'n this 'kit'?
A vacuum gauge is cheaper than a flowmaster.
Stop throw'n $$$ at it and fix'n what ain't broke.
AGAIN I'll ask, what size tires are you run'n on this jeep? What's your ring & pinion ratio?
What tranny are you using?
LG
Click to expand...

Here is a link to the rebuild kit.
AMC Jeep CJ 7 Wrangler 4 2L 258 Master Engine Rebuild Kit MKJ258F 1986 1990 | eBay

I am running 33's on the jeep on 15" wheels. Its a auto stock tranny and stock AMC20 in the rear and Dana 30 in the front.

I guess you are right I will have to check for a vacuum leak before I blow 800 bucks on the rebuild kit and exhaust.
 
Wonder where that stuff is made? :confused:
The piston size starts at .030" over bore.
33's with a 2:73 or 3:31 R&P will make the jeep gutless.
Look at changing your R&P. Leave the engine alone------
Is the tranny a 4 speed or 5 speed?



LG
 
Lumpy Grits said:
Wonder where that stuff is made? :confused:
The piston size starts at .030" over bore.
33's with a 2:73 or 3:31 R&P will make the jeep gutless.
Look at changing your R&P. Leave the engine alone------
Is the tranny a 4 speed or 5 speed?



LG
Click to expand...

I'm not to sure if its a 4 speed or 5 speed any way to tell? Also what is R&P? sorry I'm still a newbie to this site.
 
Lumpy Grits said:
:confused: How many forward gears do you shift in the tranny, 4 or 5?
R&P=ring and pinion that's housed in the differential.
You really need to get a copy of the shop service manual(MR-252)ASAP.
'84-'86 Jeep Service Manual (Body/Chassis) ? CBJeep
Click to expand...

Well its an automatic only has 1, 2 and drive. I talked to the shop just now he said he needed to tweak the carb a little more also stated that the R&P did need to be replaced as it was really worn out and recommended upgrading to a Dana 44 so might look into that. I read up on beefing up my 20 or going the Dana 44 route and still not sure which route I want to go.
 
Depending on what you are doing with your Jeep, you don't need a Dana 44 when the 20 can be upgraded for far cheaper than a 44 swap. If you plan on jumping your Jeep, you might as well skip the 44 and go right to a 14 bolt.

This shop sounds like it wants to rebuild your jeep for you...on your dime. I would be careful with these guys.

Back to the original problem. It could very well be a timing issue dropping your power like it is. But if you are running 3.31 or higher as your gear ratio and turning 33's with an auto tranny.....you aren't going to have a lot of power.
 
Get away from that shop-Did you tell'em to ck the R&P? Does it look like the diff cover was pull'd?
With an auto Transmission and 33" tires. I would strongly recommend either 3:73 or 4:10 R&P gears
I would recommend go'n to Moser 1-piece axles in you factory '20' rear end.
This will tell you how to learn the R&P ratio the Jeep was built with.
am i blind? differential code on amc 20..where?
Post #11 has the code break down.
LG
 
Lumpy Grits said:
Get away from that shop-Did you tell'em to ck the R&P? Does it look like the diff cover was pull'd?
With an auto Transmission and 33" tires. I would strongly recommend either 3:73 or 4:10 R&P gears
I would recommend go'n to Moser 1-piece axles in you factory '20' rear end.
This will tell you how to learn the R&P ratio the Jeep was built with.
am i blind? differential code on amc 20..where?
Post #11 has the code break down.
LG
Click to expand...
Thanks for the link.. They did check the R&P and said the rear had water in it and had a loose pinon. After reading all this am I think I am going to go with the 1 piece axles.
 
One other thing that hurts the 6's performance, is your 5300'+ altitude in ABQ.
LG
 
I also have a '84 and with the stock gears and 31inch tires it's a slug. I wouldn't even think about 33's with out a gear swap to at least 4.10s.
 
A little known fact about the 1984 and every other computer controlled jeep with a AMC 258 i6 / 4.2l . Most of them have bad computers that no longer do anything. When this happens you lose a ton of horsepower and fuel efficiency. The main reason you ask? Since the computer controls most of the timing the factory distributors only have 7.5 degrees of mechanical advance built in. By comparison, pre-comp controlled AMC 258 i6 / 4.2l 's have 16 to 18 degrees built in. Most people don't realize this and then dog the AMC 258 i6 / 4.2l for being an underpowered slouch. I agree they are still underpowered but with the right total advance timing your Jeep will scoot much better then it probably does now. I am building a AMC 258 i6 / 4.2l for my Scrambler right now and I threw out the motorcraft ignition system and bought an HEI unit from a 1978 Chevy C10 with a 250 inline 6. All I need to do and swap out the drive gear from my old unit and run a hot lead. Problem solved, throw out the old system and never look back.
 
Not sure I agree with "Most of them have bad computers that no longer do anything."
They have to work to pass our SMOG tests here, if you're still run'n the OEM carb setup.
I have a spare 'puter for my CJ. never need'd it in almost 31 yrs.
LG
 
Lumpy Grits said:
Not sure I agree with "Most of them have bad computers that no longer do anything."
They have to work to pass our SMOG tests here, if you're still run'n the OEM carb setup.
I have a spare 'puter for my CJ. never need'd it in almost 31 yrs.
LG
Click to expand...

You may be right in your neck of the woods but out east that little module is all but forgotten since we do not need emissions on our Jeeps anymore. I just wanted to help the OP figure out his power wo's
 
The most common power loss issues I have seen with the carb'd AMC 258 i6 / 4.2l , is incorrect ign timing and vacuum leakage.
LG
 
kaotik1 said:
I guess I was just used to have a V8 CJ I have had 2 other CJs one had a AMC 304 and the other had a 360 and never had any issues with going 55 mph down the road with 33's.
Click to expand...

Surely, that is part of it...V8 vs I6.
But the AMC 258 i6 / 4.2l only puts out about 110HP....at your elevation, you're not getting all of that...the tire height could be hurting you as well.
Quit throwing money at it...I don't know if that shop your frequenting has your best interest in mind.
Advance the timing from stock about 10*. Make sure you or the "shop" are setting it correctly.
What kind of Holley and what size jet(s)?
Slow down and take it one step at a time.
 
You must log in or register to reply here.

Jeep-CJ Donation Drive

Help support Jeep-CJ.com by making a contribution.

Help support Jeep-CJ.com by making a contribution.
Goal
$200.00
Earned
$60.00
This donation drive ends in
0 hours, 0 minutes, 0 seconds
  30.0%
Contribute
Back
Top Bottom