Replacing Manifold Gasket (88 4.0)

I am replacing the manifold gasket on my 88 with a 4.0 due to a suspected vacum leak. I idles at about 1250 and occasionally when I start it it wants to rev at about 2500.

Is it possible to unbolt the manifolds & slip in a new gasket without removing all of the stuff bolted to the manifolds? Or will this just end up causing me grief?

Chris

Have you checked everything else before going to this extreme? Have you replaced the CCV harness which connects to the valve cover? My 88 XJs owner's manual says to replace those every 50,000 miles. You also may want to pull off the throttle body, have it cleaned, and install with a new gasket.

I had a high idle problem, 1500 rpm. I had it in for service and the dealer mechanic adjusted the idle speed to the spec value. I think you could do the classic test of spraying carb cleaner around the intake manifold gasket and the base of the throttle body to see if the engine RPM does increase.

If it does then you may want to change the intake/exhaust manifold gasket. I do recommend you chase the threads with a tap to make threading new bolts in easier. You will get more accurate torques if you chase the threads. Do use a torque wrench and tighten in the order showed in the maintenance manual.

Also go to the dealer and pick up a new set of attachement hardware for the intake & exhaust manifold. My cousin told me he learned the hard way to just get new hardware at the dealer, don't reuse the old or use parts store stuff. He twisted off new stuff installing it.

Hope you don't have to go to the extreme of pulling the manifolds.

I think you should verify that you really have a leak before you go tearing the manifold off. It's a fair amount of work and you have to remove quite a bit of stuff. You can test for leaks by spraying WD40 around the suspected area when the engine is running. In my experience the exhaust is much more likely to leak than the intake. If you have the C101 connector, it can contribute to the problem you have. I've heard that some '88s have it and some don't. My '87 has it and cleaning it has mostly cured my idle speed problems. It's a major chore to clean the goo off of the contacts and I ended up removing each and every contact from the connector shell to do it. In retrospect, it would have been easier to toss the connector and solder the wires together.

In the top of the throttle body there is an orifice (air bypass) and the piston from the idle motor. With the motor running and warmed a bit, you can work the throttle a little or cover the top of the TB, partialy with your hand and watch the piston move to adjust the air some. At idle the piston makes constant small adjustments, hard to see, but noticeable with a flashlight. Could be the piston is gunked up and sticking, mine had a whole lot of semi hard deposits in there. If you clean the IAC, remove it, don´t mess with the piston (mine poped out onto the driveway), don´t get it wet with solvent, clean it with your finger nail and maybe a drop of oil.
Mine had a high idle, turned ot to be a leaky transducer for the ERG valve, blocked the in hose for the transducer, idle settled right in.
Another time it was the major vacume line, to the vacumn canister.
On another XJ, it was a leaky vacumn booster diaphram.
Another was loose intake manifold bolts. Be carefull of the studs at the end of the manifold, snap off, if you sneeze on them.
Just some things to look at, before you go to the intake manifold gasket, Be a bumber to replace it and find out that wasn´t the problem. Also a good chance to mess something up, the plastic pieces, everywhere, get old and brittle, snap off pretty easy.

CCV harness - Is that the vacum harness, if so it was new about 10,000 miles ago & is in good shape. I also cleaned the throttle body & checked the TPS when I first started dealing with the high idle.

I didn't realize the gasket was such a pain to replace so I'll do some more checking.

The IAC seams to be ok, I replaced with a used one and there was no change.
I snugged up the manifold bolts early on, in the process and again there was no change.
I'll double check the vacum system, I can't rember if I checked the EGR or not so I better look at it again
I tried using carb cleaner, wd-40 & propane to check for vacum leaks, and the engine rpm did not increase. It didn't even increase when I sprayed it directally into the intake, the engine just died. When I sprayed it around the back of the engine the engine also died, thats why I suspected the gasket. In hindsight mabey it was just wishful thinking, or mabey this is now that I've found out what a pain it is to replace.
I'd better clean or eliminate the c-101 connector as well.

Thanks for the advise & please keep it comming.

Chris

Troubleshoot by eliminating possible leaks. First pull and plug the vacuum hose to the brake booster and check the idle. If that isn't it, reconnect and proceed to check all the vacuum lines leaving the intake manifold. There is a common problem with a busted plastic hose that runs to the bumper vacuum canister. It breaks roughly under the battery. The carb cleaner technique works, but you have to be carefull not to loose your eyebrows. I learned that the hard way about 40 years ago.

There are numerous possibilities for vacuum leaks. I just troubleshot one for a friend where the EGR valve was the problem, so take nothing for granted.

I don't know if it makes a difference, but when the engine is cold it idle's at about 800 rpm.

Chris

old_man said:

Troubleshoot by eliminating possible leaks. First pull and plug the vacuum hose to the brake booster and check the idle. If that isn't it, reconnect and proceed to check all the vacuum lines leaving the intake manifold. There is a common problem with a busted plastic hose that runs to the bumper vacuum canister. It breaks roughly under the battery. The carb cleaner technique works, but you have to be carefull not to loose your eyebrows. I learned that the hard way about 40 years ago.

There are numerous possibilities for vacuum leaks. I just troubleshot one for a friend where the EGR valve was the problem, so take nothing for granted.

Click to expand...

Funny just noticed the other day, a surge at start up. Noticed it changed with the setting on the heater controls. No surge with the heater off, what next?
Just on a hunch, awhile back, I cleaned my MAT sensor, manifold air temp. idle changed and seemed not to want to hunt so much.
Might want to have a look at the throttle stop and the TB butterfly valve, possible somebody could have messed with it in the past and your cleaning of the TB, loosened up the IAC enough, to mess with the last throttle/butterfly adjustment. Adjustment of the throttle stop, messes with the TPS setting, just take a look and see if there is much/any gap between the butterfly and the wall.
TPS sensor adjustment seems to have some bearing on idle, at least mine changes some after an adjustment.
If you unplug the C-101 plug you are opening a worse can of worms, than the intake manifold. I´ve found it easier to unplug everything upstream of the connector and downstream of the connector and test for resistance through the connector. I removed one and cleaned it, never again. Only a few wires go throught the connector, that can mess with your idle. I have an extra wiring harness, that I lay out on the garage floor, along with a good wiring diagram, it makes checking the harness IN the XJ, much easier, by following and looking at the destinations on the garage floor. Primative but functional.
On a hunch, check the wires at the front end of your injector rail, that go down the front and under your motor to the knock snensor, O2 sensor and temperature sender (for the computer). Often find them frayed and/or cooked on the exhaust manifold. Could account for the idle going up with the temperature, it´s usually just the opposite, starts high and then goes lower.
You could be right about the intake, possible when it heats up it leaks worse. A cracked exhaust manifold, has also been known to possibly, affect the O2 sensor and idle, had one with high idle, after changing the exhaust manifold that idled lower, but then again, it could have been the gasket. Have heard from some guys, an exhaust manifold crack, can have a venturi affect and actually suck some air under certain circumstances, sounds logical, who knows.
If the O2 sensor is getting extra air, the idle is going up.
Last tip, if you do take the manifold off, have plan for dealing with some exhaust manifold cracks, can´t remeber taking one off, without finding at least a little crack someplace. I always take off the heat shields and throw them away, newer XJ´s don´t have the heat shielding and next time, the manifolds are a whole lot easier to remove.