Please Help: '90 XJ possible vac. leak

Please Help: '90 possible vac. leak (keeps dying sometimes won't start)

For sometime, I have been hearing a noise (kinda like a ringing in your ear). It's pretty loud. Ok, the problem is, it keeps dying on me. Sometimes it will start right back up, and sometimes not for several hours or days. A couple of days ago, I got it started. While sitting in park, I turned on the heat and it died. Does this sound like a vaccum problem or something else? How can I check for a vac. leak? Any help would be appreciated. thanks!

yes/no/ an maybe.

the easiest way to check for a leak is with your ear-but if that dont work you need a vac/compression guage- sears makes a nice cheap one.

Hook Mr guage inline between the intake manifold and the main vac line- running, it should show 13 on the guage-steady(some differ-mines 13). The guage shouldnt jump around. Now shut it off and recheck the guage. On our older jeeps it wont hold vacuum long, but it shouldnt drop to "0" immedately- if it does, theres a line open that needs fix`n.
-for pre92 jeeps-
favorite spots to check are the hardline that gos from the canister behind your front bumper/right side-to behind the battery, up to the coolant bottle and into the softline. Also, the brake check valve- trouble spots are the EGR valve haveing a hole in the diaphram, and bad expensive sensors.

On yours, my first guess would be the IAC motor is stuck open or needs cleaning. Idle Air Control.
http://autorepair.about.com/cs/troubleshooting/l/aa042603a_08.htm
thats for looky only, their tests wont work on our jeeps.

followed by maybe a bad TPS(throttle position sensor), or a vac line plugged/off the map sensor.

shoot, could even be a bad CPS. You`ll find it- happens to all of us sooner or later. Clean the IAC and Throttlebody an see what happens.

I took off the Idle Air Control, and it was clean. I still haven't been able to check for a vac. leak, because I can not get it started. Would it hurt to try and start it with the IAC discontected?

without the IAC connected, it`ll probably run rich since the airway is blocked....buttttt, Im not 100% sure if the IAC is in or out without power applied. If its out, it`ll run ultra lean.

thinking back- check to see if you have spark at all when trying to start. Thats the first of three- spark/gas/air. If you got the first two, somethings gonna happen even if its wrong.

rereading your first post-"I turned on the heat and it died"- if you had the hvac in the off position, then turned it on, the vacuume starts going to all the inards underdash- without vacuume underhood, everything that requires it then dosent have it- its static. Therefore, try starting it with the hvac off. That would eliminate underdash vac leaks. Theres still vac going to the hvac controls, just not being used.

The IAC is a stepper motor. It takes a sequenced series of pulses on the 4 wires to make it turn. It turns one step at a time, forwards or backwards.

I've tried starting it with everything off, and it cranks but won't start up. Can I still check the vac. pressure if it won't start?

no- you`ll not have any.

stick with the basics- gas/spark/air. Dont worry about the IAC yet.
list up year/model/engine info-without it, were shooting pigs in the dark, and that just makes the pigs mad.

If you got gas and no spark, check that CPS- bad cps will let the engine turn all day long untill the battery is dead. You wont get a single spark.

Check for fuel pressure at the rail- look for a bleeder valve and see if theres pressure.

untill it starts, you really cant do a vac check. The engine should run-poorly- with bad vac lines...but it should at least spit and try to start. backfire even.

Thanks for all the input. I still can't figure it out, so I'm going to put a CPS on it. From reading other threads, the CPS sounds exactly like my problem. That still does not account for the vac. leak, but I do not think that has anything to do with it not starting. The part has to be ordered, so I'll update when I get it on.

ordered?
Ive yet to go to any part house or jeep dealer that didnt have at the minimum 4 on hand. they break and go bad all the time..and sometimes..never.
Why not test yours first? you need an ohms meter anyway if you own a jeep. And if you havent pulled it out yet, try the "unplug/replug test"- works sometimes.

addemendum to the jeep owners handbook thats in the glovebox " thou shalt carry one good CPS in your journeys, or be stuck in the middle of nowhere without". And i`ll add to that- carry the tools to put it in, or cuss alot.

I carry 2.

about the only "extra" thing I dont carry is the master computer...an thats only because I havent run across one yet. the toolbox in the bed is full...thats why it dont break down.

ok, i've put on a new cps, and still didn't start. it's not getting spark, so i had the ignition coil and module tested and they're fine. do i have a pick-up coil? what about the ignition relay? it does crank, it just won't start up. i just can't figure it out.

I had similar problems on my last XJ and it was bad battery cables.

Got mopar replacements from the dealer (~$50) and never bothered me again.

i just replaced the battery, and everything looked ok, no corrosion or anything. could it still be the cables?

it is cranking, sounds strong, it just won't start.

Make sure you got a good engine ground but I'm still leaning toward the CPS circuit/wiring. The CPS itself rarely goes bad because it's a simple coil. If it gets physically damaged, it will go bad but there's nothing to go wrong otherwise.

However, the more common Renix CPS trouble is found in the wiring/connector area. I'd go back to the ECU connector and measure the CPS resistance from there. If you don't see 200-300 ohms across the CPS, wiring & connector, you should consider splicing the connector out of the picture altogether. I did and haven't had a CPS no-start in years.

There is a cam position sensor inside the distributor but it has more to do with fuel delivery than spark. But, Renix seems to stop everything if something like that is not sending proper information.

One guy recently had a starter that was leaking electrical noise into the flywheel and it was affecting the CPS signal. Bizarre, but if you can't find something else, it may be worth a look?

Leave your IAC installed and alone for now. Once you get it running we can help you with a good throttle plate idle & TPS adjustment adjustment that has been working great for me.

ck your heater a/c controls there all run off of vacme as is your windshieldwipers if they are slow look for a leak in that area