Rebuild or swap engine??

blaznjeep · 03-23-2004, 06:44 AM

I have a 4.2L that just started to make a lot of noise, sounds like lower engine. I am assuming the worst, but am trying to find a shop to take it to and have it properly diagnosed. It sounds like a bad piston/rod, maybe piston slap? It has made other noises, but it sounds terible now.

I need to weigh some options, and want your opinion.....

1) Rebuild the engine (What should I expect to pay)
2) Replace with a used/rebuilt 4.2L (Cost? Where?)
3) Replace with a used/rebuilt 4.0L (cost? Where?) No more carb troubles!!

Thanks for your advice. How risky is it to drive if it has a bad piston, rod, etc...

CanukYJ · 03-23-2004, 11:09 AM

I don't know the costs on those unfortunately, BUT I would suggest just going on ebay and start pricing a new 4.0. It's a much better engine, more reliable, more parts available. Go the 4.0 way and I think that would be money well spent in your case.

b1web · 03-23-2004, 02:52 PM

I just finished option 3. It was a long expensive road but in the end worth it. I did not do the work myself (Time, tools and expertice were lacking) so I paid about $2,500 for the labor. The engine will be anywhere from 500 to 1500 depending on year and miles (Make sure and get the harness and ECM from the same engine if possible, also make sure that the ECM has the same trans configuration you will be using (Auto/Manual). Also if you have a peugot trans you may want to replace it w/an AX-15 trans (300.00+-) if you do that add a couple hundred to replace the input shaft of your T-case (or buy them as 1 unit). Add a hundred more or so to replace the fuel pump and regulator (if you go with a 1996 or newer 4.0). You will also need to get a little exhaust work done to add bungs for 1 or 2 O2 sensors (Again depending on 4.0 year). Add a speed sensor ($120.00), O2 sensors ($60.00 each) I think thats about all the big ticket stuff.

Your gauges will be a bit of an issue but nothing a shade tree mech can't figure out.

It is allot, It is expensive, but it is definitly worth it if you are planning on keeping it for any length of time. My gas mileage is through the roof and the power is at least 1/3 better, particularly in the top end.

Find a good jeep guy who has done the swap before and work out the price for labor up front if possible, expect some headaches and enjoy the education you'll get doing the project.

My $.02 as always YMMV

zpmada · 05-05-2004, 05:13 PM

I went to my local jeep dealer that I have been told is excellent with great customer service. I had a mechanic listen to my engine because it has been making a noise. I've been told it's the valves, others said it's the crank, and this mechanic thought it was the timing chain, but said its coming from the engine so it's piston slap. He recommended using a can of "Restore" and see if that helps. Do you guys recommend anything? What will happen when my engine dies from piston slap? Will it blowup, or just not run one day. When will I know it's time to order a rebuilt engine? Will compression test tell me anything about how far i'm into piston slap and when my engine is just about to die? or does piston slap not effect compression?

Hackle · 05-05-2004, 07:20 PM

I did option number 3 myself. It cost me $2400 total and I drove it into my garage on Sat. and out the next Sat. The swap is not that hard (first time I have swappped an engine). I would do it the same way if I had to do it over.
Good Luck
BTW I installed a 98 4.0 out of a ZJ with 57,000 miles on it.

yoshisushi · 05-05-2004, 09:34 PM

For those who have done the conversion from 4.2 -to- 4.0 / AX15, if you had the opportunity to do it over, would you do it the mostly the same or do things differently?

03-23-2004, 11:09 AM	#2
CanukYJ Registered User Join Date: Sep 2003 Posts: 2,335 Profile Gallery Classifieds My Jeeps	I don't know the costs on those unfortunately, BUT I would suggest just going on ebay and start pricing a new 4.0. It's a much better engine, more reliable, more parts available. Go the 4.0 way and I think that would be money well spent in your case. __________________ My jeep has only ever killed 3 things, a cat, a deer, and my wallet. What's the problem officer? 1992 Red YJ. Lifted, locked, and loaded with beer (Disclaimer: This doesn't mean said driver endorses drinking and driving, please enjoy alcohol responsibly and do not operate any vehicle of any kind, thank you....) Thanks to the price of gas here ($4.25 a gal) I just bought a mountain bike, 0" of lift, 0" of travel, MT's, locked rear axle, oh yeah, it's the sh#$.

05-05-2004, 05:13 PM	#4
zpmada Registered User Join Date: Dec 2003 Posts: 196 Profile Gallery Classifieds My Jeeps	Piston slap-what to do to maximize life? I went to my local jeep dealer that I have been told is excellent with great customer service. I had a mechanic listen to my engine because it has been making a noise. I've been told it's the valves, others said it's the crank, and this mechanic thought it was the timing chain, but said its coming from the engine so it's piston slap. He recommended using a can of "Restore" and see if that helps. Do you guys recommend anything? What will happen when my engine dies from piston slap? Will it blowup, or just not run one day. When will I know it's time to order a rebuilt engine? Will compression test tell me anything about how far i'm into piston slap and when my engine is just about to die? or does piston slap not effect compression?

03-23-2004, 02:52 PM	#3
b1web Registered User Join Date: Jul 2003 Posts: 22 Profile Gallery Classifieds My Jeeps	I just finished option 3. It was a long expensive road but in the end worth it. I did not do the work myself (Time, tools and expertice were lacking) so I paid about $2,500 for the labor. The engine will be anywhere from 500 to 1500 depending on year and miles (Make sure and get the harness and ECM from the same engine if possible, also make sure that the ECM has the same trans configuration you will be using (Auto/Manual). Also if you have a peugot trans you may want to replace it w/an AX-15 trans (300.00+-) if you do that add a couple hundred to replace the input shaft of your T-case (or buy them as 1 unit). Add a hundred more or so to replace the fuel pump and regulator (if you go with a 1996 or newer 4.0). You will also need to get a little exhaust work done to add bungs for 1 or 2 O2 sensors (Again depending on 4.0 year). Add a speed sensor ($120.00), O2 sensors ($60.00 each) I think thats about all the big ticket stuff. Your gauges will be a bit of an issue but nothing a shade tree mech can't figure out. It is allot, It is expensive, but it is definitly worth it if you are planning on keeping it for any length of time. My gas mileage is through the roof and the power is at least 1/3 better, particularly in the top end. Find a good jeep guy who has done the swap before and work out the price for labor up front if possible, expect some headaches and enjoy the education you'll get doing the project. My $.02 as always YMMV

05-05-2004, 09:34 PM	#6
yoshisushi Registered User Join Date: Feb 2004 Posts: 86 Profile Gallery Classifieds My Jeeps	For those who have done the conversion from 4.2 -to- 4.0 / AX15, if you had the opportunity to do it over, would you do it the mostly the same or do things differently?