Rebuild or Replace

I have a XJ 4.0 HO with 150K miles. As mentioned in a different thread, I have lots of tar and gunk under the valve cover gasket. When driving, if I floor it at a stop light blue smoke blows out (oil). Then at cruising speeds while the RPM is at 1500 - 2000 the whole vehicle vibrates when I try to accelerate, but then runs smooth when I exceed 2000 RPM's. So between, the gunk, blue smoke, and sluggish hesitations / vibrations at 1500 - 2000 RPM's what is the oppinions of AF.

Thanks

Depends on the condition of the rest of the vehicle. If the rest of the drive train's in good shape, interior and exterior OK, all other "systems" work, then it may be worth poping a grand or 2 to get everything else right.

My '88 is just a small notch above being a beater. No headliner, paint's dead, tailgate panel has a large crack, driver seat ripped, but has no rust (SoCal car) and runs well. Faced with a engine rebuild (or even a swap), I'd probably replace mine. KBB value on mine is only about $1500 at best so I'd be reluctant to spend that much to fix it up.

Everything else is in good/great shape. The interior, transmission, transfer case, axels, yadda yadda. My only problem is the engine. So, I was debating weather or not to rebuild the engine, which would also mean buying a new head, or just to buy a crate (rebuilt) engine. The rebuilt would be a little more $ but the labor would be less $.

Anyone else have or had the same dilema? And could you please give me some links on rebuilt engines.

Thanks

I would say it depends on what the jeep is worth to you... My first car, I didn't care what it needed I fixed it many times although my parents told me it wasn't worth it. To them it was a 10 year old car with a ton of miles to me it was my baby, it was my first car that I actually bought. until I wrecked it... damn firestone wilderness tires.

The vibrations might be transmission shudder, the lockup might not be working right. I'd start with fresh transmission fluid for that, and also replace the Ujoints while you're at it.

As far as the crud in your engine, sounds like it is sludged up quite a bit from not having the oil changed enough. Before you go into the major expense of replacing or rebuilding it, try some of this:

www.auto-rx.com

(also www.auto-rx.net for transmission)

At 24.95 a bottle it's a hell of a lot cheaper than a rebuild. According to the website you'll need 2 treatments for the engine since over 100K.

I've seen the 4.0 motor go well past 250k, so there's no reason (barring mechanical failure) that yours couldn't do the same.


Yes, I've used the product myself (2000 Grand Cherokee, 4.0 with 90K) and it works as advertised.

Recently, I've cleaned the injectors, replaced the fuel filter, tested the fuel pressure regulator, and all is good. Also, completed a tune up but I still have my problems. The next thing I'm going to look at is the auto-rx stuff, and also my u-joints. if anyone has anything else they want to add feel free.

main problem - at 1500 to 2000 RPM's while crusing the engine and drive train stumbles & hesitates, almost like it has no power to accelerate. But if I lower the gear to get my RPM's above 2000 it'll accelerate just fine.

Crankshaft Position Sensor - if this is faulty would it cause this problem? Would the engine even run if the CPS failed? I just read that the CPS has an important role to the timing of the engine. And my intial thought was timing, but then found out you can't adjust timing on this 94 4.0 HO.

I'm going to test the CPS after work but I would like some feed back on this sensor.

The CPS tells the computer when the engine is at TDC(top dead center) so that it can fire the spark plugs in the right order and for the right amount of time. From alot of posts on here if the CPS is bad then the engine will not start because the computer does not know when it is at TDC so doesn't know when to ignite the spark plugs. While you are working on your jeep something else you could check is make sure the throttle body is good and clean and check all the vacuum lines and fittings running to the intake manifold maybe they are a little gummed up causing it at low rpm's but at higher rpm's with higher vacuum they open up enough. Another thing that you could check would be the MAP sensor. The previous owner of my sport had electrical problems (because he obviously had no idea what he was doing) and burned through the hard line that runs from the intake manifold to the map sensor so when it would idle it would be extremely rough and feel like it almost wanted to die out on me but when I unplugged the line from the map sensor and intake manifold it made the engine rev up a little to about 1k and run smooth.

Ok, I won't waste my time on the CPS, but I'll check the MAP along with all of the vaccum lines and connections. I already cleaned the throttle body and intake. I replaced the PCV's (front & back) along with the tubing. The back PCV and tube was plugged but I guess I need to double check the connection onto the intake.

Anything else other than a MAP?

Well, I tested the MAP sensor and only had 1.2 volts. So I replaced it because the book said it should've been 4 to 5 volts. That didn't fix the problem. But When I pulled the vaccum hose off of the manifold the RPM's did jump up to 1000, and of course it was smoother.

but I did want to add to my description of my problem. The hesitation is only when the engine is under load and when the RPS's are at 1500 RPM's. Start offs are good, but when cruising or going up a hill and if the RPM's are at 1500 and you press the gas the whole Jeep shakes.

The O2 sensor is new, the PCV's are new, complete tune up, cleaned injectors and throttle body, new MAP sensor.

well i could've saved alot of money if i did this first, but the good news is i have lots of new parts on my jeep now. the problem was 2 bad injectors. what i did to find out was pull the plug on them while the truck was running. starting at cylinder one and working my way back to #6, everytime i pulled the plug on a injector while the engine was running the idle would drop. makes sense. well i found 2 of 'em that had not changed the idle when i pulled the plug. i replaced them and now its all smooth. all 6 cylinders are cranking now.

fyi ... i replaced the fuel pump and the pressure test was the same. just like the book said .... 31 psi with vaccum line on the regulator.

well i could've saved alot of money if i did this first, but the good news is i have lots of new parts on my jeep now. the problem was 2 bad injectors. what i did to find out was pull the plug on them while the truck was running. starting at cylinder one and working my way back to #6, everytime i pulled the plug on a injector while the engine was running the idle would drop. makes sense. well i found 2 of 'em that had not changed the idle when i pulled the plug. i replaced them and now its all smooth. all 6 cylinders are cranking now.

fyi ... i replaced the fuel pump and the pressure test was the same. just like the book said .... 31 psi with vaccum line on the regulator.

Sweet; good catch.