High idle rpm issue?

Having high idle issues with my 96 cherokee 4.0, have ruled out vacuum leaks and it's showing no codes. Idle is at 1200 rpm on tach, but when hooked up to diagnostic machine and smog machine it's showing the idle at 750, I'm thinking thinking the ECM is bad. Anyone have any thoughts on this?

ECM/PCM failure is very highly unlikely, and should only be considered after thorough and in-depth diagnostics and trouble shooting of the more common and more likely suspects for the symptoms.

Are all the tune-up parts fresh ? Visually inspect for loose wire plugs and loose vacuum hoses or fittings. Test the vacuum and PCV system. Clean and test the Idle Air Controller.

600-700 is about normal for the Cherokee idle speed, wondering why you think there is a problem ?

Reason I suspect ECM is because the obd2 port is telling the diagnostic tool and the smog machine that the engine output is 750 rpms and the actual output is 1200 rpms

JoeDirt57 said:

Reason I suspect ECM is because the obd2 port is telling the diagnostic tool and the smog machine that the engine output is 750 rpms and the actual output is 1200 rpms

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That's impossible, if the ECM thinks the engine is running at 750 rpms, then the engine is running at 750 rpms, it's impossible (short of something other than the ECM providing fuel and spark) for the engine to run beyond the rpms the ECM is telling you.

Now if the ECM says that the target idle rpm is 750 and the it's also saying the actual rpms are higher than that, then you have a vacuum leak/base air flow issue which is not an ECM issue by any means.

Appreciate your help on this, and again while plugged in @ the obd2 port it reads 750, but the engine in fact is running @1200, I'm clueless as to what would cause that other than a shorted ECM. Every once in a while while driving and in gear it will actually kick down to the proper idle. When I bought it check engine light was on, downstream 02 sensor, changed it and notice the IAC was unplugged. After that had the high idle but no check engine lights. Had a exhaust leak so changed the manifold, still high idle. I've checked for vacuum leaks . So the ECM thinks it's at the right idle but it is not

You need to put a scanner on it that reads live data. You're gauge may be failing, put a separate RPM meter on it to confirm the PCM readings. Codes are only thrown when something is completely out of spec, but if your PCM see's something it will try to compensate for it.

Yeah that's what I've done both scanners that have been on it read live data and there is no doubt that the rpm data is wrong at the obd2 port and the tach is right. So thinking pick a part for a cheap comparable pcm, guess I should check the pcm thats in it and make sure it's actually the right 1. Thanks for the input

I'm telling you with 110% certainly that the PCM is not the issue here, this sounds like a faulty tach issue or your scanner is giving you false data, neither of which will be fixed by replacing the PCM.

Let me walk you through how this all works, as the engine spins the PCM reads the crank position sensor and the cam position sensor. This sensor data tells the PCM how fast the engine is spinning and when to fire the injectors and the ignition coil. The software in the PCM processes this data and streams it out over OBD2 which gives you the data you're reading on your scanner. Then the PCM sends a pulse for every time it fires the coil on a pin out of the PCM which is read by the Tach. It's impossible for the engine to spin faster than what the PCM is telling you and have it still run because the spark and the injection events would be happening at the wrong times, it's also impossible for the PCM to be triggering the tach output pin any faster than what it's driving the coil.

Also even if the tach is reading 10,000 rpms at idle, that shouldn't effect the smog test and if your smog guy is failing you for a faulty tach, please let me know who he is, so I can report him to the B.A.R. and have his smog license revoked.

I'm not talking about anything reading the OBD, but an actual RPM meter that reads off the coil.

Smog guy has no problem running the test! The problem is he has to run a test at idle then a no load at 4000 rpm. Actual rpm has been verified at the coil as matching the tach. Ok if he runs a no load which the rpms are determined off the smog machine that puts the engines rpms , we'll I'm not sure where it would put. But past the point of engine failure. It sounds like you completely understand the emission system, if it's not the pcm what would you try next? I've replaced the IAC, the TPS, The exhaust manifold, I having actually used a vacuum test gauge but visually checked all connections including the map.

I would trust the OBD data more than an instrument cluster gauge. I think you have two simple options for testing the current tachometer accuracy, find a junkyard instrument cluster and swap it in, or wire in a standalone aftermarket tachometer.

Is there an MSD box installed on your jeep?

No MSD box, all stock

When the smog tech is hooking up to get an RPM signal how is he doing it? Is he hooking the inductive clamp to the #1 plug wire? or is he plugging in over OBD? or is he using the inductive pickup on the coil itself?

Stupid question: has it always done this, or is this obd2-vs-tachometer thing recent?

Any chance someone did a full-gauge swap on this in the past and put in a 4-cyl tach instead of one for a 6-cyl (assuming I'm remembering right and there's some sort of "resistor mod" or similar that needs to be done to match things up)?

He is going over the plug wire, i just picked it up and am looking at things now, I was mistaken about the no load smog, it's done at 2500 rpm

Going over the plug wire

Decided it has too go to the shop.

JoeDirt57 said:

Decided it has too go to the shop.

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I hope you're taking it to G&G Auto Repair, talk to Richard (He's a member here)

Dropped it off today, parked next to cherokee , 1 was a shop truck. I've used G&G for other things , great shop