"new" 87 cherokee 4.0 starting issue

hey guys! pretty new here, but not new to jeeps by any means. however, this is my "newest" . anyway its an 87 pioneer cherokee with the 4.0. I have used as many search words as I can think of to look at past threads dealing with this issue but still I have no clear answer. anyway, here goes: I just purchased this cherokee and as stated it is equipped with the 4.0. The problem is it hesitates when starting and idles fairly low (according to my tach around 500-600) the previous owner stated that the jeep had sat (climate controlled) for about 2 years with little or no regular starting. The whole jeep is cherry but this starting problem bugs the hell out of me. it takes a good 5-7 seconds of cranking before starting. When it does fire, it revs to about 1500 then drops steadily to idle(again, about 5-6 hundred rpm) stumbling is not an over riding issue, but it does occur and I notice that it will ALWAYS stumble (and nearly stall) if at idle I snap the throttle wide open then shut it quickly. in general, the idle is decent but it is "fairly" rough in drive, especially with the A/C on when it seems the engine is working harder. also, the previous owner was nice enough to give me the past emissions slips and in the 2 years it was tested (01 and 03) nearly every area had higher (worse) readings. I did all the usual stuff, replace the air filter, oil added new high octane gas, hopefully to counter act with the aging gas that was in the tank. I really love this truck and being in college I dont have unlimited funds...but If you guys can tell me what you think the posssible causes could be that would at least give me a solid start. Im already assuming the injectors are at least a little dirty. aside from that im pretty well stumped on what to check. any ideas, or past experiences will be a great help. thanks guys!

I would ensure the plugs and wires are good.....actually, I would just replace them, along with the cap & rotor and O2 sensor. Also, thoroughly clean out the throttle body and AIS motor with t-body cleaner. You might want to get the fuel injectors cleaned using one of the pro systems.

Oil added? Does this mean you didn't drain and fill it? If so, do change it as it's way past due if it sat idle for 2 years.

As for the long starts, it could be the CPS. Rags, who used to post here a lot and was a Jeep tech, mentioned that you could "adjust" the CPS by very carefully prying the sensor so that it's slightly closer to the flywheel. There's a voltage reading associated with this procedure but I don't know what to look for. Try a search. I know the camshaft sensor in the distributor has been known to cause long starts. Hopefully others will chime in with their ideas.

I had similar problems from time to time with an 87, and it usually helped to get out there with contact cleaner and dielectric grerase and clean up every single electrical connector I could find. The 87 has a big connector on the firewall, through which all the electronic stuff passes, and this can cause problems, as well as things like the one to the TPS. Stalling and stumble off idle were helped by this a few times. I also found that a slightly misadjusted TPS could contribute to the stumble, but I wouldn't fuss with that until I'd cleaned things up.

The various electronic signals in this thing are very faint at best, and it seemed to need a little tweaking a couple of times a year in this corrosive climate.

A weak or failing CPS is a good possibility in this vintage. If your crank time is very long, but it cranks evenly, that's a prime suspect. Remember, though, that the Renix system is designed to crank for a minimum of four engine revolutions before it fires, so even a perfect one will not start the way other engines do.

You can check the CPS with an ohmmeter. It should show 125 to 275 ohms hot.

If the hard starting is accompanied by erratic cranking that seems almost like hydrolocking, look to the distributor and its initial indexing.

If my typing si bad, please forgive - I squashed my "d-e-c" finger on a rock yesterday and it's challenging to type with a bandaid on.

Matthew Currie said:

Remember, though, that the Renix system is designed to crank for a minimum of four engine revolutions before it fires, so even a perfect one will not start the way other engines do.

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Yeah, this bro's given good suggestions. Renix cranks a lot longer before ignition than any vehicle I've ever owned before. The good news is that the rod/main bearings have a pretty good flow of lube oil before firing pressure is ever exerted on the bearing shells.

You should clean the TB (throttle body) and particularly the IAC (idle air control) stepper motor passage. After that, do a good HOT throttle plate idle adjustment with IAC fully closed and disconnected. Followed-up with adjustment of the TPS (throttle position sensor) per FSM (factory service manual).

Also, check/clean all connections as mentioned but pay attention to the injector connectors too. They are notorious for holding moisture and corroding to the point of intermittent contact causing miss-fire conditions.

The fact that it idles up at start and falls back down tells you/us that the IAC is working but if it continues to act slow after cleaning it may need replacement. Change the 02 sensor too. About $52 from Kragen/AZ last month for me.

Learn the Renix MPI system and you'll be in good shape. It's really pretty decent except there's no CEL (check engine light) to let you know when it needs attention. Gotta go with how it runs and learn to qualify components with a DVM.

thanks guys for the help. I went out this moring actually as was looking at how the XJ acted in certain situations.I noticed that when I started the truck this morning it fired after about 5 seconds and idled right around 800 but slowly dropped back down to about 500. it sounded perfect right at 800rpm. then, just to experiement I put the jeep in gear and gave it just a short touch of gas...and it stumbled then lurched forward. I only notice that it stumbles when I give it throttle too quickly at idle speeds. I know the egr is ok because when the diaphram encounters excessive VAC. it kills the engine. so now that I know thats good ill check your suggestions. so where exactly are these sensors etc.you guys talk about. Idle Air Control, Crank Position Sensor etc etc. when I get time I will clean the throttle body...I just have a strange suspiscion it has something to do with some sort of engine control. thanks!

scase86 said:

thanks guys for the help. I went out this moring actually as was looking at how the XJ acted in certain situations.I noticed that when I started the truck this morning it fired after about 5 seconds and idled right around 800 but slowly dropped back down to about 500. it sounded perfect right at 800rpm. then, just to experiement I put the jeep in gear and gave it just a short touch of gas...and it stumbled then lurched forward. I only notice that it stumbles when I give it throttle too quickly at idle speeds. I know the egr is ok because when the diaphram encounters excessive VAC. it kills the engine. so now that I know thats good ill check your suggestions. so where exactly are these sensors etc.you guys talk about. Idle Air Control, Crank Position Sensor etc etc. when I get time I will clean the throttle body...I just have a strange suspiscion it has something to do with some sort of engine control. thanks!

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IAC is on the side of the TB. It's simply a bi-polar stepper but you need electronics knowledge to get into operational details about the driver circuit. No big deal unless you're interested in manipulating it. Cleaning is the most important step because the TB area never gets fuel to wash things down.

TPS - Again, located on the TB. Even if you replace it, there's an adjustment procedure for the Renix TPS but you must have a good base throttle plate idle before doing that adjustment or you'll just have to do it again.

CPS - No adjustment there but if you wanna look at it, it's mounted about 11 o'clock on the flywheel housing. It's a very simple Hall-effect sensor with nothing to "burn-out", "wear-out" or "fail" except if it's encasement fractures and it becomes contaminated with moisture. However, the wiring connector(s) often the fault with this component and '87's are more notrious than later versions. My '89 connector was found at the firewall behind the cylinder head just on the drivers side. The connector's since been replaced and I've never had a crank but no-start CPS failure since.

EGR - I haven't had much trouble here. It either works or leaks vacuum. If I suspected vacuum leaks, I'd check it out but a low problem component based on my experience. 260k miles. Owned since 83k. Head's never been off yet.

Check the HB (harmonic balancer - front pulley) for excessive runout and seperation. That doesn't have anything to do with your driveability issues but age takes it's toll on that part and it would suck to have it fail on the road/trail.

Change the 02 if unkown how old it is. Bad, weak, inoperable, it will affect
idle and fuel mileage through a too-rich mixture.

Sure, new plugs, wires, cap, rotor can help. Clean TB and make certain the CCV (crank case ventilation) is in 100% condition. Might need to remove the valve cover and replace hose assembly to get CCV right but it's a very "balanced" system and it does not tolerate leaks and clogs very well.

What's the thermostat doing? Does it warm quick and up to about mid-range on the gauge?

Connectors, connectors, connectors on Renix. Clean-em right and use dielectric grease. Most of my sensors are still OEM but the connectors needed some rework to remain reliable.

PM me if you can use a FSM in PDF.

My best guess would be 02 sensor (especialy if the idle changes/lowers, a lot, when the motors hot), if the 02 sensor is the old type with the heat shield, it should have been replaced long ago.
Another trouble spot for rough idle is the distributor cap, my caps seems to go bad (corrosion) more often than the plugs need changing.
An occasional treatment with a brand name injector cleaner (gas additive), also seems to help some.
Old gas is bad stuff, I´ve got a fun truck, that usually sits all winter. Every spring, I have the old gas, gunk something up, usually takes a couple of tank fulls (and some injector cleaner) to get it cleaned out. The gas filter is invariably full of junk.

awesome info guys. XJXJ I could use the factory service manual on pdf if you have one...that would be awesome. I will shoot you a PM. as far as o2 sensor I will have to take a look at that. Im pretty sure it hasnt had any major overhauls or service in the past couple years since it was sitting. but I do need to get an emissions so the 02 is probably a good idea. also I just noticed tonight that an electrical connector comming off the distributor is broken to the point where I can see the inside of it. since I know this part has seen some water/solvernt from cleaning the engine bay, I wonder if this could be a problem? does anyone know what this connector off the distributor is? it is a type of pigtail. seriously guys I really appriciate this help!

Cam position sensor (it has other names though), have heard it will mess with starts if bad. I forgot and left mine unplugged once, it didn´t seem to do much (if anything) on mine.

8Mud said:

Cam position sensor (it has other names though), have heard it will mess with starts if bad. I forgot and left mine unplugged once, it didn´t seem to do much (if anything) on mine.

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I never tried running my 87 without one. I think it's more critical on the chrysler system, though.