XJ's same old idle problem with a twist
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XJ's same old idle problem with a twist
Hello all; I have read all the common idle problems and have had them myself except this time it got a strange idle problem in my 91 XJ 4.0L that gets very specific. I replaced the old fuel injectors with new 4 hole injectors, also installed the factory heat shield along with heat shields for each injector. After first staring it up the engine had a dead miss on more than one cylinder. After a few restarts and running a little Seafoam in the intake and in the gas, it seemed to straighten out and pretty much ran smooth. Then I notice a strange problem with the idle fluctuating. If i run the idle up to 1500 RPM it will hold steady for exactly 10 seconds then will rev up exactly 500 RPM more to 2000. As soon as it hits the 2000 mark it will immediately drop the 500 RPM back down to 1500. At that point it starts the cycle again every 10 seconds. There is a bit of rough idle at it's normal idle speed, just enough to be irritating. I know this has to be a sensor of some type. My only thought was that when I Seafoamed it that I might have damaged the O2 sensor and now the PCM is hunting for a signal. Any ideas of such a regular type of idle fluctuation ? Any suggestions will be appreciated, Thanks Max.
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Year: 1992
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Engine: 4.0L L6 PowerTech (stock)
If the old fuel injectors had to inject more fuel, then the air intake valve would have throttled more open to allow more air - I would think.
Maybe you just need to clean the gunk from you idle air control valve. Show us a before and after when you post the results.
Maybe you just need to clean the gunk from you idle air control valve. Show us a before and after when you post the results.
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No the A/C is not engaged. There is no cycling of system. This is just the engine running. I have everything else turned off including the radio. I have chased idle problems for 28 years on this thing and can usually get it under control. Over the years I have cleaned or replaced the idle control unit, throttle body, TPS, intake manifold, complete air cleaner system, MAP sensor, MAT sensor, all vacuum lines, Computer control unit and even replaced the exhaust manifold when it developed cracks like most of them did. But I understand about the A/C unit being on since it completely resembles what the A/C does to the idle when engaged. It was fine except for the normal Heat Soak that happens to the older injectors. That's why I changed the injectors and added the heat shields. After that is when the condition started happening. This is what lead me to believe that it had to be some sensor reacting to the new injectors. If you think of anything please let me know, Thanks, Max
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I will clean the Air control valve again. I cleaned it last summer, including the hole it goes into. That was probably the fourth or fifth time I have cleaned it in the last 28 years. The Jeep might just be getting too old for everything to function as it should anymore. After all it has over 300,000 miles on it. If the Air Control valve is trying to control the idle to it's proper specs, would it do it on 10 second intervals or would it be more of a constant RPM change. Being that I can hold it at 1500 RPM for exactly 10 seconds and then the system just increases it to 2000 RPM and then as soon as it hits the 2000 RPM it goes right back to 1500, for another 10 seconds and starts the exact cycle again makes no sense to me. Does the Air Control valve operate the system if it is in the range I am talking about? I've never seen any information on exactly what RPM range the Air Control works, so I'm at a loss of knowledge on that part of it. I'm just like most XJ owners on this subject, in that I am totaly lost whe n it comes to chasing the dreaded idle demons. I'll let you know if cleaning it does any good, Thanks, Max
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I removed the throttle body and cleaned it and the AIr Control Valve. Rechecked all vacuum hoses for cracks and replaced any that looked suspect.This is before and after cleaning. After reinstalling everything, it still has the same problem. There was a very small difference in the amount that the idle increased when holding it at 1500 RPM but not enough to make any kind of difference in the problem. Still got that O2 sensor on my mind. By the way I did clean out the hole that the air valve fits into along with cleaning the throttle body.
Last edited by Maxspeed; 03-13-2019 at 10:22 PM.
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My idle was cycling similarly on my 90. I changed the 02 sensor for other reasons, and lo and behold, the idle smoothed out. Suppose it was seeking the right mix but couldn't find it with the old dead 02 sensor. (I had tested it's function, (not just the heater), and found it was dead) 87 -90 does have a different type of 02 sensor.
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Flintstone: That was my first thought when all this happened. So I guess i'll pull the O2 sensor and see whats what. Here is an update from this afternoon. I took the XJ to it;s emissions test today. Last year my HC: was 219 and the CO: was 1.18, today the HC shot up to 680 and the CO went to 9.82....Whats up with that? Just from going from one hole original injectors to new 4 hole injectors. Maybe the brand I got was not the best. They were supposed to be 703's but they were sold by Navi Move. I'm betting I just installed a set of Chinese injectors.
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