Shop cant figure out no start condition - JeepForum.com
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post #1 of 9 Old 02-16-2020, 03:35 PM Thread Starter
93zjer
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Shop cant figure out no start condition

Background: 1988 Jeep Comanche 4.0
-Has always ran well once warmed up
-Past 6 months it seemed to have a somewhat high idle while warming up.
-A few time it started and then died after a few seconds. Ran fine upon restarting.
-Failed to start once. Battery was slightly weak so it was charged over night. Started well the next morning. (I do not believe the battery was the issue, t was far from dead, just turned over a little slow I thought.)
-Engine died while idling. Would not start. Turns over strongly.

Had it towed to a reputable local shop. They have not been able to diagnose the problem. I gave them permission to throw a crack shaft position sensor at it. They say it will idle if they cause a vacuum leak. Otherwise it will not start/run. They also say that it is running very rich.

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post #2 of 9 Old 02-17-2020, 07:31 AM
87MJTim
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If you have not already done so, check out www.cruiser54.com for his tips on improving the Renix Jeep.

The high idle could be a vacuum leak. Check the vacuum line for the MAP sensor.

The no start could be many things. Is fuel pump working? Perhaps the ballast resistor is going bad. Unplug the wire from the BR and connect them together.

It dies at idle? Does it run if you give it gas ? Check the IAC (on the throttle body). Could be bad or just dirty.

Cranks but no fire: perform a test of the CPS. See cruisers tips.

Another good reference guide is Lunghd http://www.lunghd.com/Tech_Articles/...iagnostics.htm

Good luck

Tim

87 Comanche, 4.0, AX15 (formerly BA10/5), D44
84 CJ7, 4.0 (93 XJ) AX15 (Formerly T5)
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post #3 of 9 Old 02-17-2020, 11:34 AM Thread Starter
93zjer
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Quote:
If you have not already done so, check out www.cruiser54.com for his tips on improving the Renix Jeep.
I'm well aware of Cruisers Renix tips but this MJ is not the one I'm keeping long term (I own 2).

Quote:
The high idle could be a vacuum leak. Check the vacuum line for the MAP sensor.
The high idle was only while warming up. In my experience they run like **** with the MAP sensor unplugged (but it isnt)

Quote:
The no start could be many things. Is fuel pump working? Perhaps the ballast resistor is going bad. Unplug the wire from the BR and connect them together.
How could it start and run (with a vacuum line unplugged) if the ballast resistor or fuel pump are not working?

Quote:
It dies at idle? Does it run if you give it gas ? Check the IAC (on the throttle body). Could be bad or just dirty.
No, not sure where you're getting that. The closet thing to that I wrote was "A few time it started and then died after a few seconds. Ran fine upon restarting."

Quote:
Cranks but no fire: perform a test of the CPS. See cruisers tips.
At this point I have to wonder if you read a single line of my post. They replaced the CPS. "I gave them permission to throw a crack shaft position sensor at it. "
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post #4 of 9 Old 02-17-2020, 04:22 PM
87MJTim
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When you turn the key to start, the starter relay bypasses the fuel pump relay and ballast resistor and directly powers the fuel pump. With key in run, the fuel pump relay send the power through the ballast resistor to the fuel pump. The engine will run long enough to burn off the fuel pumped to the fuel rail. A bad BR will not power the pump.

As for the idle issue, if the IAC (also called the Air Stepper Motor) is not opening enough to let air bypass the closed throttle plate. That could also account for it stalling at idle. Does the engine run if you continue to give it gas?

Tim

87 Comanche, 4.0, AX15 (formerly BA10/5), D44
84 CJ7, 4.0 (93 XJ) AX15 (Formerly T5)
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post #5 of 9 Old 02-17-2020, 06:20 PM Thread Starter
93zjer
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I see, so the ballast resistor could be bad, but not the fuel pump, since it can idle without dying as long as a vacuum line is unplugged.
Only few times did it die a few seconds after starting, and that was spread out over several months. Currently it will not run without a vacuum line unplugged, not at all.

No, it will not run if you give it gas. It wont even start unless a vacuum line is unplugged, and then it does not die, whether you give it gas or not. But it does run rich according to the shop.

I need to get it towed to my house (like I should have done to begin with) so that I can fix it myself. At the time I was thinking it was the fuel pump and I had almost a full tank. Given that and just being busy with work I thought it made sense to let a shop fix it.
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post #6 of 9 Old 02-17-2020, 06:46 PM
87MJTim
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Those links have pdfs of both the 88 Electrical Wiring diagram and the Renix Fuel Injection system. Both of these are very helpful in diagnosing the Renix Jeeps.

Tim

87 Comanche, 4.0, AX15 (formerly BA10/5), D44
84 CJ7, 4.0 (93 XJ) AX15 (Formerly T5)
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post #7 of 9 Old 02-17-2020, 07:03 PM Thread Starter
93zjer
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Quote:
Originally Posted by 87MJTim View Post
Those links have pdfs of both the 88 Electrical Wiring diagram and the Renix Fuel Injection system. Both of these are very helpful in diagnosing the Renix Jeeps.
Thanks! I am only familiar with the 91 and later 4.0 engines.
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post #8 of 9 Old 02-17-2020, 07:20 PM
87MJTim
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Renix (87-90) is completely different from HO (91+) Jeeps. Just saying.

Tim

87 Comanche, 4.0, AX15 (formerly BA10/5), D44
84 CJ7, 4.0 (93 XJ) AX15 (Formerly T5)
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post #9 of 9 Old 03-05-2020, 01:23 PM Thread Starter
93zjer
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I wanted to post how this resolved for anyone who read the thread in the future.

Turns out the shop had replaced the CPS with a new defective part. Then instead of using a meter or oscilloscope to properly troubleshoot, they threw parts at it without authorization. Coolant temp sensor and O2 sensor. Now dont get me wrong these part were absolutely bad, and my truck rums much better now, but I already know it could run better if I replaced some sensors. I have two MJ's and this one is being sold soon.

Then they tried dot charge me almost $800 for diagnosing and replacing a bad CPS.

I told them the O2 and coolant temp sensor could not possibly have been keeping the truck from running and that I'm not paying for the senors. I told them I'd pay $400 (which I think was generous). The owner agreed (with an attitude like I was ripping him off) and stormed out of the office. I paid the receptionist and took my truck home.
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