philwarner

I have not yet tried starting with the IAC in place but disconnected. I did note that when I pulled the throttle body the new IAC was backed off what looked like quite a bit. I removed it and it measured about 1.140 from the mounting flange and the tip of the pintle and .430 between the body collar and the underside of the head; it was definitely not closed. I've cleaned the throttle body and the bore for the IAC was already pretty clean from spraying carb cleaner in it from the back and swabbing it with Q tips when the throttle body was on the engine but it is now scrupulously clean. I have one screw on the MAF which was stuck but not tight and the head was a bit messed up where someone probably tried to remove it previously and stripped it a bit. I was finally able to clean it up with a pick and get a fresh T25 bit to get a grip, but it turns so hard I thought it would snap so I sprayed some liquid wrench under the head and am letting it soak overnight. I can put the other screw in to hold it, but I did want to clean it before putting the throttle body back on. I am going to try the original IAC when I do, but it occurs to me to observe it in the throttle body before I put it back on to see how it moves and how much it backs off from fully closed.

Noah911

This is what I recall:

The IAC pintle finds its end-point when the Jeep is shut down. When you turn the ignition key off the pintle is supposed to go to its end-point for calibration reasons of the next start, and then it backs itself off going into a 'park' position. On the next turn of the key to run, the IAC is supposed to pull back into its 'home' position.

I am not 100% sure on this though? [edited] -does not come from official sources and there is conflicting information.

philwarner

I found the 'IAC Live training' video at . Kind of folksey, but very good information. Thanks for the video link.

Apparently GM uses a different pin configuration than Chrysler since they were measuring adjoining pins for winding resistance. Guess I need to do the test light test on the connector once I get things back together to make sure the PCM is sending 12V pulses, but since mine did move the pintle both ways I expect it is sending pulses (but maybe too many backing it off too far?).

I'll look for vacuum leaks after I try watching the IAC in the throttle body before I bolt it down. Hopefully the MAF screw will cooperate tomorrow.

philwarner

No luck on the stuck MAF screw, so I gave up and put the throttle body back together using the 2nd MAF screw to hold it, but before I installed the throttle body I set it bottom out with the IAC connected so I could see (and video) how the IAC pintlel moved. It did extend quickly to the closed position where the seat stopped it and then quickly backed up again to a length of 1-1/8" from the mounting flange to the tip of the pintle (.409 from the lip on the body to the underside of the head.) This was my original IAC and it was moving both directions and apparently OK. So far so good.

This is the "at rest" pintle position after it pulled back from closed;



However, now that the throttle body is freshly cleaned, my "idle" has gone up to 2000 RPM and still surges, the tachometer needle still blips, the gas gage still has a mind of its own, the "check gages" light is on most of the time, and the exhaust spits black soot. I still have no Check Engine light and no codes except when I tried starting with the IAC disconnected which caused the Check Engine light to come on and set a P0505 code for " Idle Control System Malfunction ". The idle was still 2000 RPM with the IAC disconnected or reconnected. I put a vacuum gage on the throttle body with the main vacuum line disconnected and had 19 inches Hg at 2000 RPM, and when I added a T and reconnected the main vacuum line and the gage to the throttle body, the vacuum is still 18 to 19 inches Hg at 2000 RPM, so I don't think the high idle is due to a vacuum leak.

I am running out of possible causes besides the idea that the replacement PCM is dodgy and moving the IAC way too far causing the high idle and somehow affecting the tachometer and gas gage and apparently the fueling as well. I can't think of how the Crank sensor or cam sensor would cause high idle and they seem to be working if it starts and runs. If folks have any other suggestions to check, I'd appreciate hearing them.

One thing I learned today was that if you are having someone help in diagnosis, make sure she understands that "turn the key on" does not mean "try to start the engine". I was fortunate that no fingers, hoses, or tools were near the belt when I was watching the IAC pintle move, and that the camera was safely in front of the Jeep.

Noah911

Are you able to return the newly purchased IAC?

Noah911

I can only give you my thoughts for consideration at this point. I am going to go out on a limb and tell you exactly what I think is going on with it.

Water is a byproduct of gasoline combustion. The black soot coming out of the tailpipe is probably just water mixed with carbon. Water usually comes out when the engine is not hot enough like when starting it. During these few days of testing you probablly have not gotten it up to a hot enough temperature for long enough to fully evaporate the water. I do not think it is an issue.

The fuel gauge I believe has been sort of a red herring to me. The Jeep is starting. It is not stalling out. Your scanner can now communicate with the ZJ having a replacement PCM installed.... Inside of the fuel tank is a level sensor that has a float which looks like a bicycle pedal. The float rises and the resistance of the electrical contacts increase to give the gas gauge a reading. The electrical contacts of this sensor use friction against each other, and these are most likely worn and wearing out because of age, and is the reason for the fuel gauge errors.

Are you willing to kind of take a shot in the dark? I am going to go against some of my own advice a little. Below is what I would do as an attempt to fix the main high erratic idle problem at this point.

Even though it appears to test normal, I would take the chance and replace the TPS. The throttle position sensor was one of the very first most likely suspects in my mind. I would replace it.

I am going to go even further away from my own advice, and recommend to replace the sensor with an inexpensive BWD Advanced Auto Parts TPS (instead of spending the $90 on a Mopar TPS). With 20% off the BWD comes to around $25. It comes with a lifetime warranty should it fail early due to it being a cheaper lower quality aftermarket sensor.

To me.. There has been some rather sufficient testing done. Spending the $25 for the BWD TPS would be comparable to me buying a new tool, such as a torque wrench or something, to do my own repair work. I would be utilizing the BWD TPS as a testing measure, and it would be worth it to me.

If you are able to return the IAC? This is basically sort of a free part too, to verify the current TPS is not the issue.

For some reason, I do not feel as though the problems you are describing are due to a fault of the PCM.

Maybe at the point where if the BWD TPS did not fix the issue.. I might consider another PCM?

(edited)
If it has not been done? I would spray everything possible, all around every hose, and every last gasket sealing area in the engine compartment with carburetor cleaner, starter fluid, or an unlit propane torch to make sure there is no vacuum leak.

philwarner

I was going to try connecting the TPS from my 96 Cherokee Country which is a slightly different part number on the O'Reilly's web site but looks very similar, but the mounting screws on it were stuck tight like the GC's MAF screw, so I took the GC's TPS loose fo it could "unwind" and my Ultragage said TPS absolute position was then 3.92 VS the 11.0 it reported before. When I started the engine the RPM was surging around 1500 RPM instead of 2000 and the vacuum was 13 inches Hg VS 18 inches Hg, but everything else was the same; tachometer dropping out and back up, rough running, sooty exhaust, gas gage varying from 1/2 to E and back, Check Gages light on, but no Check Engine or codes set. If I shifted to drive the RPM dropped to 1000, but it was still running really rough and I just moved it a short distance to a more convenient parking location and shut it down. I put in for a refund from the eBay seller and will look for another PCM.

Here is what I told him as the reason. (Don't know why this pastes so large, but it did it three times now)

The PCM you sent for my 96 Jeep Grand Cherokee ZJ 5.2 V8 does let me start the engine, but it "idles" at 2000 RPM, the tachometer keeps blipping, the engine runs rough and surges and it spits black soot from the exhaust, the gas gage keeps changing from 1/2 tank to empty and back again, and the Check Gages light is on most of the time. I've cleaned the throttle body, replaced the Idle Air Control, replaced the Throttle Position Sensor, and checked for vacuum leaks with none found, and still the idle is way too high and the engine runs rough and surges. No codes are set and no Check Engine light comes on. If you have a good PCM, I'd be OK with a replacement, but if you don't have a good one please send me a return shipping label and refund the $150.49. BTW, the PCM I received was missing three of the four screws that hold the cover on and had a broken off screw jammed in one of the mounting lugs that I could not get out and I had to mount it with just two screws.

Going through the refund route there was an option to print a return label and a packing slip to go in the box so I did that and will use the box it came in. Now I hope we don't get blown away in the coming storms and tornado warning; It is going to be a strange summer around here.

Noah911

The storms... It might be interesting? We already have this many tornados and the earliest tropical depression with cyclonic activity on record since satellite monitoring began in the 1960's. Hopefully there is no Dorian this year.

Storm One-E formed well south of Mexico's Baja California Peninsula on April 25, becoming the first ever April tropical depression and the earliest formation of a tropicalcyclone on record in the Eastern Pacific Ocean.

philwarner

One last test:

I exchanged the new IAC for a new TPS and installed it. Today I back probed the new TPS center signal wire and had Signal voltage with key off of 8.3mV and key on closed/idle 0.605V, and WOT 3.79V. The new TPS absolute position was 13.3% at closed throttle and 73.3% with accelerator floored. Note that the original TPS first was 11.0% and then 10.6% at closed throttle and I apparently did not write down its Abs TP with accelerator floored; its signal voltage was0.552V at closed throttle and 3.42V at wide open throttle. Spec says signal voltage should be between 0.26 and 0.95 volts closed and less than 4.49 volts wide open, so both appear to be good, but the new one is closer to the middle of the spec. I needed to move the Jeep to leave room on the right side to pull the PCM again. Turning the key on, the low fuel light was on, the Gage said E, the Check Gages light was on, the Abs Throttle Position was 13.3%, the temp was 72F. It started and I shifted to drive to move it and then back in neutral it ran 1400 to 1500 RPM surging, shaking, spitting, and smoking gray smoke with the vacuum varying 12 to 13 inches Hg in time with the engine’s one second cycle surges. While running the fuel came up to 1/2 and the low fuel light went out, but the Check Gages stayed on. I shut it off and then turned the key on again without starting and the Abs TP % was changing from 13.3 to 12.9 and back to 13.3 and then 12.9 over and over in 2 second cycles.

I think I am definitely down to the PCM as the high idle cause because it had the same symptoms when only the vacuum gage was connected to the bib on the throttle body and I don't see any other vacuum connections that could be leaking. Are there any other vacuum connections?

Noah911

I have another suggestion. This is where my thoughts went to after hearing the results from today.

I would want to think at least one step ahead of myself when considering a PCM. Say you get another PCM, and it ends up not being the reason. What would you be left with? A vacuum leak is the obvious answer.

Have you considered the fuel system? How old is the fuel in the gas tank? I would just do some quick diagnostics of the fuel system prior to purchasing another PCM (or exchanging this one).

(Copy/paste):
A badly clogged fuel filter will reduce fuel pressure enough that the ECM must open the fuel injectors more to maintain the same amount of flow. Once the ECM has effectively "pegged" the fuel injectors into the open position, fuel pressure rapidly rises, which shoots more fuel into the engine than it needs. The ECM compensates by shutting the injectors, resulting in another fuel pressure drop and renewal of the surging cycle. Fuel injectors contain very fine mesh filters, which can just as easily clog and cause the same sort of surging.

Bad Gasoline

After spending some time in storage, gasoline will begin to react with the oxygen around it and lose its potency. This oxidization process is essentially combustion in very slow motion: fuel combines with oxygen to produce carbon dioxide, water and contaminants like nitric oxides and loose carbon molecules. Once the engine begins ingesting this "pre-burnt" gasoline, its ECM reads the exhaust temperature as a "lean burn" (too much air) condition. In attempting to adjust, the computer injects more bad fuel, which essentially douses the flame in the engine's combustion chambers. Once the ECM detects a lack of flame, it backs off fuel injection, runs lean and must repeat the surge/stall cycle over and over again just to keep running.
(End copy/paste)

You have a scanner and it will not read the CEL for you? Another try at the CkPS to disconnect it and see of the code will be read by your scanner maybe is worth it, just to see?

Noah911

I don't know if this helps? It is for a 1995 ZJ.

philwarner

It is a 5.2 V8, so there are the three Ms on the diagram that represent separate vacuum bibs from the one on the throttle body? The one marked P? That "P" is the one I connected my vacuum gage to before I added a T there and reconnected what I thought was the main vacuum line. I'll have to go look for those and see if there are any missing vacuum lines. I do have a disconnected and capped vacuum line that runs to the cruise control.

On the bad gas idea, I added 5 gallons of fresh 100% gas to the tank when I first tested the fuel pump by jumping the socket for the relay, and I disconnected the fuel line at the rail and pumped a half gallon of gas into a clear container and it was relatively clear and did not have the bad gas smell. Reconnecting the fuel line I was seeing 50 PSI at the fuel rail which is in spec so it was not a bad fuel pump as I had suspected. That is what lead me to check for the 5 volt output of the PCM and replace the PCM when it was missing.

I can try disconnecting the CPS to see if it throws a code although I had already seen the P0505 code set when I left the IAC disconnected and started the engine, so I don't think it is a matter of my Ultragauge not being able to read codes but that the PCM is not setting/retaining any codes, although I still can't figure why the check gages light stays on even when the gages are all reading normally, and it does not set any codes.

philwarner

I traced the vacuum lines and had the large hoses (the Ms on the diagram) to the PCV and the brake booster that appeared intact and the "P" line from the throttle body that ran along the right valve cover to the back of the engine and across to the left side over the brake booster and forward to the "duty cycle purge solenoid" and then from it (according to the vac diagram on the cowl) to the evap canister and also to the fuel tank "rollover valve w/orifice". This line appeared intact also. At the rear right near the firewall was the other disconnected and capped vacuum line with a capped T in it that runs to and forward along the right inner fender to a T to the cruise control bellows and then to a line that goes forward under the battery box to a device in the bottom with some kind of plunger that gets depressed by the battery.




Upon further searching I located an open 1/4" hose bib on the right side of the intake manifold that was hidden by the larger PCV hose. It had nothing connected to it and has never had anything connected to it since 2011 as long as I've owned the Jeep. I removed the T I had added at the throttle body and connected my vacuum gage to this bib



The vacuum diagram on the front cowl says this "M" bib is "to climate control, speed control, 4X4 axle (when equipped)". It has not been connected to the cruise control nor to the climate control nor to the mystery device in the battery box in the 10 years the Jeep was running OK. Unless there was another T that is missing, It was apparently not connected to the 4X4 axle either.

With the new found vacuum "leak" now plugged with the vacuum gage, I started the engine from cold and had a 1400 to 1500 RPM idle, surging engine, Vacuum varying around 14 inches Hg in time with the RPM surges, and a sooty exhaust. No DTCs were set or pending. Later, after cranking with the CPS disconnected to see if any codes were set (none were), I restarted the engine and it was back to 2000 RPM "idle" and 20 inches Hg vacuum and when I shifted to drive (foot firmly on brake) the RPM dropped to 1000 and the vacuum to 14 inches Hg. Switching on the A/C made no difference in the high idle.

I see a sensor device connected to the intake manifold at the right front. Would this be measuring vacuum? The Ultragauge had a measure called MAP PSI which I set it to show and it was reading 2.03 to 2.18 at "idle", the second value came after revving it to 2500 and letting it fall back. A later restart showed the MAP PSI to be 1.13 at the high idle.

Noah911

Did you find out if that was the MAP sensor located there on the manifold? I would have to look up the 5.2L sensor layout. Someone else tested for a bad MAP like this below if you wanted to try and test it, and use this as a way too?

(copied/pasted)
Did some testing today, backprobed the MAP signal to PCM:

When it was working good: the idle was about 1.45volts. But when the stattering started, the idle was 2.78v. Gave it some gas and the engine didnt rev up, the MAP was still outputing 2.78v etc.

When all was good - the voltage changed with every rpm change.
Now just waiting for the sensor.
(end copy/paste)

There is an EGR valve on the manifold.

(edited)
I don't know a MAP would do something like what is happening with yours. I guess a bad MAP can read rich or lean, or anywhere in-between?

It is sounding even more like a PCM to you now?

philwarner

Noah, I backprobed the center signal wire of my MAP (red/white) and with key on engine off it had 4.49 Volts (5V = 13.9 PSI?); The UG said MAP PSI was 4.50. I started the engine and at a surging 1500 RPM, MAP voltage was 1.96 volts (2 volts = 7 PSI?) and the UG said MAP PSI was 2.03. At a foot held 2500 RPM the MAP voltage was 1.6 volts (1.5V=5.7 PSI?) and UG MAP PSI was 1.74. Letting it idle down again to 1500 RPM, MAP voltage was 1.98V and UG MAP PSI was 2.03. The MAP PSI reported to the UG by the PCM does not agree with a graph of MAP voltage VS PSI from https://injector-rehab.com/shop/mapsensor.html . Another source said MAP PSI should be 14.7 with the engine off, but that is at sea level. The pressure at 1500 feet is 13.91, so 4.49 volts is darn close to 4.5 and 13.9 PSI so my MAP sensor must be reading OK, but the interpretation of the voltage by the PCM is off.

I note that the 96 GC ZJ wiring pdf says the speedometer, the tachometer, the voltmeter, the fuel gage, the temperature gage, the oil pressure gage, and the warning lamps all get signals from the PCM via the CCD buss. (The ABS warning lamp get its signal from the anti-lock brake controller or CAB, and the evap leak detection pump system is on California models only (hence the loose 4 pin connector by the air filter?), and there should be an intake air temp sensor which I do not see (Perhaps the reason for the loose 2 pin connector near the inside of the air filter box?).

BTW, can someone tell me what that vacuum device in the bottom of the battery box is supposed to do? Would that somehow be associated with battery temp?