I was given my '87 xj, previous owner(s) frustrated with many problems.

Fixed a lot, mostly electrical, and now it starts, runs and drives pretty good, but it wants to "idle" at about 1800 rpm. I guessed it must be IAC, TPS, or vacuum leak. Replaced entire throttle body yesterday, no change, now reckon it's a vacuum leak. This thing has a vast and complicated lot of vacuum bits, (2 fairly complicated harnesses), so I thought Id ask if there are any known vacuum problems to check first...

 

It does sound like you are on the right track. Make sure that all of the vacuum system is in good shape. Start at the vacuum ball behind the front bumper passenger side and work you way back. With a little luck you will find a cracked or broken hose and it will take care of your problem. If not move on to the TPS. Let us know what you find
Good Luck.

 

Should have mentioned in the first place, '87 XJ w/AMC 4.2 six, and renix Done all the recommended ground fixes..

Is this the engine that is improved by replacing the stock cylinder head with the MoPar head? Does the renix stuff work with the MoPar head, or do the FI electronic bits have to be replaced too?

 

Eh? Someone put a carbureted 4.2L there? Renix fuel injection was only on the 4.0L.

You probably do have a vacuum leak. Among other things. Check this thread out: https://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

 

No, just fumblebrain in the post, it's a 4.0 AMC 6.

Most of the problem is that the Jeep has a bad (leaky) booster, and thanks for the help, all..

 

Did you get the high idle fixed?

 

I took some vice grips and pinched closed the hose to the booster, idle dropped to just under 1000 rpm. pulled off the brake vacuum hose and plugged the spud, this morning the Jeep cold started and idled at 850 rpm.

The manual says that the 4 liter with stick shift should idle at 750 rpm, I reckon I have 100 rpm of further leakage to find. Ether around the injectors produced a small change in idle, so I guess I may have some leakage there, too, but not going to fool with that 'till I install the Neon injectors with new O rings..

 

Brian was over on another thread where I explained 703's are not ideal for Renix. (OK, but not ideal). The 703's will be rich while its on default's in open loop. My take on why the 746's are so smooth is that a longer pulse-width results in smoother atomization. The 703's will provide the same amount of fuel in closed loop, but with a shorter burst at the higher flow rate.

 

If the injectors that had the rev drop were the #5 or 6 you may want to look at the back three intake/exhaust bolts being loose( two on top and one on bottom).

 

Agreed. I got tricked the other way around. Thought it was the manifold when it was the O ring. Tip: the manifold gasket will hold a little water between it and the head. If that get's sucked in on starting.....

 

Put some 746s in it and don't look back.

Here's a proper vacuum diagram.

 

Was #2 &6, I think. A lot of stuff back by the firewall, may have been manifold in that neighborhood, I couldn't see real well back there.