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  #1  
Old December 23rd, 2016, 19:31
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montanaman montanaman is offline
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'91 4.0L motor not starting

Hey :

My '91 4.0L motor isn't starting. It turns over with no problem, but won't start.

Several years ago, I noticed it took longer to start, then about a week ago, it took even longer. Now tonight it isn't starting at all. It fired once, but wouldn't actually start.

I seem to remember that this might indicate faulty or dirty sensors on the intake manifold.

Any ideas? Help me out brothers.

Thanks!

Last edited by montanaman; December 23rd, 2016 at 19:35.
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  #2  
Old December 23rd, 2016, 22:09
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Re: '91 4.0L motor not starting

Check for spark first. If no spark, then suspect (test) the CPS.

If you have spark, then suspect no fuel. Check the fuel pressure or presence of fuel under pressure at the test port on the fuel rail.
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  #3  
Old December 24th, 2016, 00:03
go1lum go1lum is offline
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Re: '91 4.0L motor not starting

91 check the ballast resistor on the drivers inner fender if you have one. Its a ceramic resistor with two wires running to it that can be clipped and spliced together with no I'll effects.
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Old December 24th, 2016, 01:08
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Re: '91 4.0L motor not starting

Thank you for your ideas.

I don't think it's an electrical issue. The spark is fine. The ballast resistor doesn't even come into play until after the motor has started. If a ballast resistor goes bad, the motor will start fine, then die after a second or two. During the starting phase, the ballast resistor is already bypassed by the ignition switch. So I don't think it is either of those problems.

What are the sensors on the intake manifold? ... I remember that when they get dirtied up, it takes longer to start, and eventually won't start. Is it the MAP sensor? I wish my memory was better.

Normally, I'd just go out there and start taking them all out and cleaning them, but it's about 10 degrees here, and I'd like to have a game plan first.

Thanks for all your help, and please keep the ideas and thoughts coming.

Thanks!

Last edited by montanaman; December 24th, 2016 at 01:18.
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  #5  
Old December 24th, 2016, 05:05
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'91 4.0L motor not starting

If you have spark then it sounds like you should check fuel. Especially with the slow decline in start up.

Idle air control (IAC) maybe what you are thinking of, you can just take the throttle body off to get to it. I've changed them with out pulling the TB, but honestly if it's got spark then the fuel is where I'd check.
The other things like map, tps, Iac all would affect the running & starting but not prevent it really.

If you have to replace the fuel pump go with the OEM set up and a Bosch pump, best quality for the long run. Other aftermarket pump set ups may work fine for your needs, after decades of dealing with our xj it's the only way I'd go after trouble with aftermarket quality.


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  #6  
Old December 24th, 2016, 19:11
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Re: '91 4.0L motor not starting

Yep, sounds like no fuel pressure, or the injectors are not opening.

If that passes as good, then the IAC or the relay AS? relay that allows the ECU to reset the IAC (?) may not be resetting on shut down and thus letting enough air in, or an injector is stuck open and flooding the engine.

Have you checked the spark plugs for proper gap? If they are too old/worn they could need re-gaping.

I have had a bad/dirty Intake Air Temp sensor cause no starts and flooding and high idles, that caused no start issues.
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  #7  
Old December 25th, 2016, 14:18
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Re: '91 4.0L motor not starting

Merry Christmas to everybody. I went out last night and it started. This is really weird. At this point, I'm stumped. I'm posting this extra info, hoping it will make things more clear for someone who knows more about this than I do.

Thanks!
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  #8  
Old December 25th, 2016, 16:18
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Re: '91 4.0L motor not starting

Just some added info:

- As a reminder, the '91 4.0L motor is HO (not Renix)

- After it started, it ran a little rough for about 10 seconds, then settled into smooth running.

- It still takes longer than normal to start.

And another question: Is there another name for the throttle position sensor (TPS)? I can't find it on Mopar parts websites.

Thanks!
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Old December 25th, 2016, 19:55
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Re: '91 4.0L motor not starting

Its never been called anything but a TPS Throttle Position Sensor.

There was little change in the programing and operation of the HO engine controls in the 91 HO changes.

The sensors are all the same style/operation. Connectors are different and the TPS is different, but what they do, how they fail, and why, and the symptoms are nearly identical. Some changes in wiring, fuses, relays, and the EGR valve was deleted. O2 seonsor changed electrically, but the basic dignostics of what kind of sensor failure causes what symptom is unchanged. CPS changed electrically. But still serves the same function and causes a no spark when it dies or starts dying.

Bad TPS will not cause a no start. I have started mine with it disconnected. The CPS, IAC and MAP seem to be the critical sensors for starting, and in 91 the dizzy sync sensor was added to that list I think of needing to work to get a spark.

Quote:
Originally Posted by montanaman View Post
Just some added info:

- As a reminder, the '91 4.0L motor is HO (not Renix)

- After it started, it ran a little rough for about 10 seconds, then settled into smooth running.

- It still takes longer than normal to start.

And another question: Is there another name for the throttle position sensor (TPS)? I can't find it on Mopar parts websites.

Thanks!
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #10  
Old December 25th, 2016, 20:46
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Runnin'OnEmpty Runnin'OnEmpty is offline
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Re: '91 4.0L motor not starting

Based on the intermittant starting getting worse, a possibility might
be a cracked/split connector hose at the fuel pump. Jeep used a
lower grade hose than what should be on there,(SAE R6 vs R10 or
above). Matters are made worse with todays ethanol based fuels.

The split hose allows fuel to be pumped back into the tank, thereby
lowering fuel pressure to the engine.

The fuel pump can be accessed without dropping it, by driving the
rear tires on ramps and working underneath. The pump assembly is
angled to the front so it's relatively easy to remove.
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  #11  
Old December 25th, 2016, 22:12
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Re: '91 4.0L motor not starting

Quote:
Originally Posted by Runnin'OnEmpty View Post
(SAE R6 vs R10 or
above).
How can I tell if I'm getting an R10 fuel hose? Is it printed on the hose itself?

Quote:
Originally Posted by Runnin'OnEmpty View Post
The fuel pump can be accessed without dropping it, by ...
Are you saying I can get the pump out without dropping the fuel tank? I never knew that. And were is the fuel check valve located? It's possible mine is bad.

Thanks.
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  #12  
Old December 25th, 2016, 22:13
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Re: '91 4.0L motor not starting

Thanks for all the answers. You've given me some good things to look at. I tried it again today, and it started up again with no real trouble ... a more cranking than it should take, but other than that, it was fine.

I'll break out the FSM and brush up on the stuff you've talked about. I just wish there wasn't a foot of snow on the ground!

I'm tempted to not mess with it anymore, now that it's starting again, but it's possible I'll get stranded somewhere with an xj that won't start.

Thanks again.
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  #13  
Old December 26th, 2016, 08:32
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dan1977p dan1977p is offline
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Re: '91 4.0L motor not starting

Does it fire up faster if you cycle the key a few times to prime the fuel system? I didn't see any answer to fuel pressure question and you can at least confirm you're going down the right path before pulling the fuel pump
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  #14  
Old December 26th, 2016, 16:20
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Re: '91 4.0L motor not starting

All irrelevant if you have checked the fuel pressure at the fuel rail first (attached to the fuel injectors) by testing the fuel pressure at the fuel test port with a test gage. If it is low, it could be the pump, or the hose he mentions or the FPR regulator on the fuel rail or leaking injector(s). Alsothe fuel filter can be plugged IF the fuel pressure is low. It should be between 29 and 39 psi.

Quote:
Originally Posted by montanaman View Post
How can I tell if I'm getting an R10 fuel hose? Is it printed on the hose itself?



Are you saying I can get the pump out without dropping the fuel tank? I never knew that. And were is the fuel check valve located? It's possible mine is bad.

Thanks.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #15  
Old December 26th, 2016, 17:13
steveC steveC is offline
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Re: '91 4.0L motor not starting

Quote:
Originally Posted by montanaman View Post
Just some added info:

- As a reminder, the '91 4.0L motor is HO (not Renix)

- After it started, it ran a little rough for about 10 seconds, then settled into smooth running.

- It still takes longer than normal to start.

And another question: Is there another name for the throttle position sensor (TPS)? I can't find it on Mopar parts websites.

Thanks!
Offhand, it sounds a little like it might have flooded. After it sat awhile and the excess fuel evaporated, then it started OK. It's happened to mine a couple of times in the 12 years I've had it, usually in cooler weather when it stumbled and quit after the first try.

TPS sensors show up under fuel/air on a lot of parts references. Standard TH143 or Delphi SS10482 are common crosses, I don't know the Mopar PN offhand.

They're pretty easy to check with a meter, if you need to.

First, I'd want to know what the fuel pressure is when you kick the ignition on. It may bleed down overnight, but should kick up to about 20psi when you first turn the key on. That's been my experience, anyway.
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