'88 intermitent problem.

'88, 4.0 auto.

I got an intermittent problem where the XJ will suddenly idle rough and will not go above 1,000 RM. Previous owner gave me recent reciepts from a dealer who changed MAP, TPS, O2 sensors and cleaned TB. She said that helped for a few months then it started happening again, but not as often. The dealer could not duplicate the problem.

About a week into my ownership it happened to me. I was able to limp it into my shop, the mechanic unplugged the vacuum line that goes from the TB to the bracket located high center of the cowel inside the engine compartment (is that the MAP sensor connects?) The engine ran smooth and continued to run smooth after he reconnected the vaccum line. He doesn't really know what to tell me.

So today it happens again. The first time it cleared up before I pulled over. The second time I pulled over, uplugged the electrical connection at the bracket (engined died) wiggled the vacuum line, restarted the engine and went on my way with no problems.

Knowing what to wiggle is helpful, but I am planning on turning this thing over to my kid in a few months and I don't really want him (new driver) to worry about having to pull over in traffic and wiggle something to keep going.

So....what do you all suggest?

It sounds like the Mass Airflow Sensor is bad. Or the port where it connects to the throttle body is partially glogged. Maybe a crack in the vacuum line. This sensor sends a reading to the computer. the computer than adjusts the fuel mixture accordingly. Try cleaning the line. Unplug it at both ends and spray some carb cleaner through it.

Crank Position sensor

cygnus58 said:

Crank Position sensor

Click to expand...

Concur - I'd start there. RENIX can work without some sensors (badly, but it can run,) but it will not fire without the Crank Position Sensor signal. Ain't gonna happen - no two ways about it.

Typical service life of the RENIX CPS seems to be ~150Kmiles, and be careful when you install the new one (you can end up with the leads hitting the #6 header tube, melting through the jacket and insulation, and shorting out intermittently. Don't ask how I know...) Fortunately, if that happens, to you, you can splice around the melted leads with a bit of 18/2 SJOOW, throw some heat sleeve over it, and you're on your way again (don't ask how I know that, either. But, it did get me off the side of the freeway and on my way again in about ten minutes...)

5-90 said:

Concur - I'd start there. RENIX can work without some sensors (badly, but it can run,) but it will not fire without the Crank Position Sensor signal. Ain't gonna happen - no two ways about it.

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Wait, but when the problem occurs the jeep continues running. Just very badly. How is the CPS related to the vacuum line I mentioned above?

Where is the CPS located? What does it look like, how much does it cost and what is involved in replacing it? Where do you get one?

I don't mind changing it out for piece of mind and to eliminate it as part of the problem, if it is easy/cheap enough.

I have had a problem at 3800 rpm exactly, consistantly. Another thread the guy had a 1000-1500 rpm problem. I had another issue while trying a few for spares - one would barely idle, ran rich, and unplugging the MAP vacuum also affected it slightly. They can cause no start, stall, timing issues, and I found out, poor fuel mileage believe it or not..
Theres got to be a link with pics...
Mine was 42 bucks, OEM.
I used a 3/8 ratchet with about 15 inches of extension and an 11mm socket.
Take out the top bolt first as not to drop it in the bell- housing.
Replace it last.
If you face the engine, look down at the rear drivers side of the intake, in the cavern below the MAP sensor you spoke of, shine a light down in. Two inconspicuous bolts and two wires leading up to a plug.
If you fix it yourself it's cheap enough, and it is one of the most common "let you sit dead by the road" sensors in the Renix. Hopefully that is the problem
Since I just replaced mine, it does give some piece of mind. But the clock is ticking on the new one.....

Cygnus58,

You switched topics in the middle here I think, but kept calling it an MAP sensor (I think?), while you described installing a CPS sensor but you never mention the CPS sensor itself. You need to clarify what you are trying to say, this poor guy has a steep enough learning curve with out trying to install a MAP sensor where the CPS goes (?). Maybe I am miss reading it, but if it confused me, I can only imagine where it left him.

cygnus58 said:

I have had a problem at 3800 rpm exactly, consistantly. Another thread the guy had a 1000-1500 rpm problem. I had another issue while trying a few for spares - one would barely idle, ran rich, and unplugging the MAP vacuum also affected it slightly. They can cause no start, stall, timing issues, and I found out, poor fuel mileage believe it or not..
Theres got to be a link with pics...
Mine was 42 bucks, OEM.
I used a 3/8 ratchet with about 15 inches of extension and an 11mm socket.
Take out the top bolt first as not to drop it in the bell- housing.
Replace it last.
If you face the engine, look down at the rear drivers side of the intake, in the cavern below the MAP sensor you spoke of, shine a light down in. Two inconspicuous bolts and two wires leading up to a plug.
If you fix it yourself it's cheap enough, and it is one of the most common "let you sit dead by the road" sensors in the Renix. Hopefully that is the problem
Since I just replaced mine, it does give some piece of mind. But the clock is ticking on the new one.....

Click to expand...

Sorry but he mentioned how the mechanic unplugged the Map sensor and I thought he described where it was located. I then described the CPS in a location relevant to the Map.
He asked why the Map vacuum would affect it if it were the CPS, and I referenced how when in one instance the Map vacuum did in fact affect my idle problem yet the CPS was the culprit.
Hope i didn't confuse you bl87xj...

The 87-90 Jeeps, refered to as Renix Jeeps, did not have or use mass air flow sensors, they used a MAP (Manifold Absolute Pressure) sensor combined with an Intake Air Temperature sensor (IAT) and the Computer (ECU) used their data to estimat mass air flow from those two sensors.

I have noticed that the MAP sensor vacuum tubing that connects to the throttle body on mine can come loose of the throttle body very easily if bumbed. Mine back fired once while starting and it blew the throttle body tube right off of the throttle body and made it run terrible, Shook violently.

Don't know if cleaning that line would make any difference on the Renix MAP sensor tube as there is no air flow in that tube.

pokeyxj said:

It sounds like the Mass Airflow Sensor is bad. Or the port where it connects to the throttle body is partially glogged. Maybe a crack in the vacuum line. This sensor sends a reading to the computer. the computer than adjusts the fuel mixture accordingly. Try cleaning the line. Unplug it at both ends and spray some carb cleaner through it.

Click to expand...

cygnus58 said:

Sorry but he mentioned how the mechanic unplugged the Map sensor and I thought he described where it was located. I then described the CPS in a location relevant to the Map.
He asked why the Map vacuum would affect it if it were the CPS, and I referenced how when in one instance the Map vacuum did in fact affect my idle problem yet the CPS was the culprit.
Hope i didn't confuse you bl87xj...

Click to expand...

bl87xj,

The engine will barely run if the map sensor tube in the throttle body gets loose enough to leak vacuum.

What all of us here are saying on the CPS is that it has not been replaced, and goes out much more frequently than the MAP sensor. cygnus was describing where the CPS is and how to get to it to replace it.

There are some good threads here on the CPS, three letter searches do not work here, so use CPS* or Crank Shaft Position sensor in a search for threads on the CPS and how to replace it, blow by painful blow.

Also you need to check and clean every little two bit ground wire under the hood, as they can also cause hell in the 87-90 renix jeeps. There is a great thread titled "ReniX files" with my long sad story of wire wiggling for two years before I finally broke down and cleaned ALL my grounds.....

So loose dirty ground wires, a dying CPS, old loose sensor connections like the TPS, or loose Map sensor vacuum lines are usual suspects.

well said

cygnus58 said:

Sorry but he mentioned how the mechanic unplugged the Map sensor and I thought he described where it was located. I then described the CPS in a location relevant to the Map.
He asked why the Map vacuum would affect it if it were the CPS, and I referenced how when in one instance the Map vacuum did in fact affect my idle problem yet the CPS was the culprit.
Hope i didn't confuse you bl87xj...

Click to expand...

Sorry to be picky (was just trying to be helpful and specific), but you said "they" at the beggining of the third senstence, right after discussing the MAP in the first two senstences, and never once used the word CPS anywhere in that post (even though I thought I knew what you ment). In other words replacing "they" with "the CPS" would have made it so much clearer and unambiguous.

Its easy in the rush of typing, especially when we get tired, to leave out a critical word, I know I have done it.

I thought you ment CPS, but I did not want to put words in your mouth, and assume what you ment. Someytimes we get so use to our short hand talking expert to expert here we sometimes forget the original questiononer may still be trying figure out where the engine or oil dipstick is, LOL!

Hopefully he can follow our ramblings now.

no worries, ecomike.
I just hope bl87xj can fix this without paying the ridiculous labor that shops charge.

Ecomike said:

I have noticed that the MAP sensor vacuum tubing that connects to the throttle body on mine can come loose of the throttle body very easily if bumbed. Mine back fired once while starting and it blew the throttle body tube right off of the throttle body and made it run terrible, Shook violently.

Don't know if cleaning that line would make any difference on the Renix MAP sensor tube as there is no air flow in that tube.

Click to expand...

speaking of that tube, how do you remove it?
mine seems very tight in the throttle body..

It's a press fit on the 87 Jeeps. Maybe the prior owner glued it in? The second hole/insert is just a plug, perfect spot for contact cement?

cygnus58 said:

speaking of that tube, how do you remove it?
mine seems very tight in the throttle body..

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