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Discussion Starter · #1 ·
Hey all,
So I come to the greatest gathering of mopar minds around to (I hope) solve my problem. I have a 98' Jeep Grand Cherokee Limited 5.2L V8 Magnum. It was a SUV that I picked up for cheap due to a blown engine. Well since then we have had the engine replaced with another 5.2L Magnum engine and a smattering of new parts, plugs, coil, air filter, oil, crank sensor, battery, rear main and that about all I remember. So now it starts and idles just fine. Even revving the engine in park yields no signs of anything going awry. However after about 6~ mins of driving it under normal load it starts misfiring and hesitating badly. I plugged in my code reader that has live system monitoring and drove it around. What I found out kinda has me a bit stumped....When the misfiring starts the MAP sensor starts jumping around in its " HG reading. It will fluctuate from 8" to 15", meanwhile I am cruising going the same speed and throttle position. Before it starts acting up the oscilloscope function is very steady in vacuum then goes into pulse width so to speak when the MAP acts up. However....I have a 98 ram with the same engine and a known good MAP that I swapped into the jeep today but to no avail it still has the same problem. So what should I look into? It acts like it has a vacuum leak that randomly occurs after 6 mins and the MAP is responding to the loss of vacuum. Wires look fine in the connector but I cleaned them and gave it some dielectric grease as well.
 

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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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Same PCM? Do you know if the engine you installed is a '98-newer 5.2L engine?
1992-1997 5.2L Magnum long block is a different engine than the 1998-2002 5.2L Magnum long block. I don't specifically know what is different, but the long blocks are not interchangeable with each other in the Mopar reman catalog. This may be significant.
6 minutes may be about the time it takes to go from open loop to closed loop operation. What are the O2 sensors and injector pulse width numbers doing when the misfire/hesitation transition occurs?
Any fault codes being generated?
If you disconnect the MAP, then start and drive it, the PCM will approximately calculate the MAP value from both the TPS and engine speed values. It will light the 'ck eng' light and set a MAP fault, but that can be erased after the test.
See how it drives like this. Sometimes an engine will run better with a calculated value (by disconnecting the sensor) than a wrong value.
 

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Discussion Starter · #3 ·
Original PCM with JEEP. I have no known info about the year of the replacement block. It was sourced from a salvage yard that did not have the best records for their donor supply sad to say. I also forgot to mention in the opening that my tinkering today included cleaning the throttle body and the vacuum tube connected to the MAP so I would drive it 6 mins find out it was hesitating then I would return and clean another connector or check another part. So it was in closed loop during the tests. I parked it for maybe 5 mins each time then drove it again, my scanner never showed it going back to open loop. each time it would drive fine for roughly 6 mins then start showing symptoms. Both O2 sensors are reading values around .7, however come to think of it I think one did dip considerably when the miss started, but I interpreted that as the MAP giving incorrect values to the ECM which stated adding or lessening the value of fuel which caused the miss. Short term and Long term fuel trims were also going haywire LT was maybe around 10 on the steady cruise at 50mph I believe before the symptoms but honestly I would need to focus on those numbers again to give you a better range of values. I dont have a stand alone oscilloscope to probe the injectors to see pulse width unfortunately. And no fault codes being triggered....which is really weird given the misfire.
Ill try to unplug the MAP tomorrow and see if the base calculation for the TPS and Engn Speed will make the jeep run better.
 

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The O2 sensor values should be jumping fairly rapidly between about 0.2 volts and 0.9 volts once warmed up (entered closed loop). With the O2 heaters and hot exhaust flow, they should warm up fairly quickly. If O2 sensor values change slowly or not at all, then the sensors are likely failed. The PCM depends on the O2 sensor values as a fuel mixture 'feedback'. The PCM will then lengthen injector pulsewidth for more fuel or shorten injector pulsewidth for less fuel.
Some scan tools show injector pulsewidth in msec. I did not mean for you to try and measure it with a scope.
Long-term fuel adaption or fuel 'trim' is built up over time as the PCM 'learns' the fuel mixture required by the engine to run at best fuel calibration. It can be + or - up to 33%. Generally it shouldn't run more than about + or - 8% unless there is a rich or lean issue. Positive trim values are adding fuel to compensate for a lean condition and negative trim values are taking fuel away to compensate for a rich condition.
It can be reset along with other adaptive values, fault codes and OBDII readiness status with a battery disconnect.
Short-term fuel adaptives or trim may be more useful for diagnosis here.
Unburned oxygen going past the O2 sensor (e.g. - from a misfire) will be seen as a lean condition and more fuel will be added (longer injector pulse width or 'on' time). If the misfire is being caused by carbon-fouled (black) spark plug tips, this will make the misfire worse as the plug won't be able to burn off the carbon layer. Spark plugs that are allowed to heat up have a 'self-cleaning' tendency to burn off any fouling with the proper mixture balance.
I use a small Mity-vac handheld pump to test the vacuum response (i.e.-signal voltage) of a MAP sensor with the ignition on. Sucking on a hose will also work. This will usually confirm that the MAP wiring and the PCM interpretation of the MAP is working OK. By disconnecting a vacuum hose on a running engine and letting it suck air should also give a response (leaner) as would a shot of aerosol solvent over the air horn (richer).
If your MAP sensor vacuum reading (in HG) is showing a lot of fluctuation on a poorly running engine, it may very well be telling you the truth.
The spark plug tip color can also tell a story of too rich or too lean.
 

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You don't say if any codes were stored, whether it was a code 13, or O2 codes such as 51 or 52.
Given that it only happens when transitioning into open loop, I'd suspect that one or more O2 sensors is bad, and that the MAP vacuum fluctuating is not the root cause, but rather the result of a bad O2 sensor disrupting smooth engine operation and producing erratic vacuum by way of the engine misfiring and almost stalling. This seems to be confirmed by the readings you get on the O2 sensor.
 

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Discussion Starter · #6 ·
So pulled the spark plugs today and they look fine honestly. But they only have maybe 50 miles on them anyway so its kinda early for a lot of buildup to form. The tip shows more normal operating conditions slight brown tinge but no black fowling. So I took it out again and tried to monitor the values of the ST Fuel trim LT fuel trim and the O2 sensors. The O2's fluctuate between .2xx to .9xx when its not acting up. When it really started to buck the O2 sensor #2 which i assume to be post cat was at 1.10+ and the ST fuel trim would jump to around +14 to +33. Which is kinda weird...02 is reading rich but fuel trim is showing lean? Im sorry I didnt get a chance to disconnect the MAP and drive it around on its base calculations for TPS and RPM. The heads up display on the vehicle information screen showed that the coolant temperature sensor was faulty when I first started it up but went away as soon as It showed the fault. So I had a coolant sensor handy and was changing it out when the Coolant temp sensor snapped in half inside the intake manifold >.>. So now Im having to use an easy out on that sensor then run the disconnected MAP test after its been replaced. Which will be sometime later this week.
Bob Lincoln: Yesterday there were no stored codes even after multiple trips around town with it backfiring fairly frequently. However today I did see a check engine light when I started it up. The codes were engine misfire P0300 all the way through P0308 so all 8 were misfiring. And P0340 no cam/crank position signal. However both the Cam and crank sensors were replaced with known good units from another vehicle. And its actually transition into closed loop not open loop that the faults occur however interestingly when I was looking at the live data while driving my code reader showed that during the misfires it would occasionally go into open loop very briefly then go back to closed loop which is having me wonder even more if perhaps the coolant sensor is indeed faulty even though my code reader did not pick up a code for it. 02 sensors are indeed crucial in drive-ability and I have yet to rule them out even though they seem to be fluctuating between the correct range for most of my drive cycle.
Here's another question for you both..Internet searches are abound with people with similar problems reporting their ECM to be at fault that they are notoriously "cheap" in their construction and go bad very often. Do either of you have any say in this? Most 2nd gen dodge owners I know of with the 5.2L have never had a faulty ECM this goes for me as well with my 98 Ram. I would imagine the ECM between Dodge/Jeep to be very similar.
 

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The misfire will drive the PCM rich because it sees oxygen in the exhaust as a sign of being lean.
Have you verified good fuel pressure (about 50#) when the problem occurs?
I haven't found the JTEC PCMs particularly unreliable. They seem to hold up fine. If it was OK before the engine went, why would it go bad now?
Keep diagnosing.
 

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Discussion Starter · #8 ·
Great point. It only occurs when driving around it refuses to act up while revving the engine in park so I dont know of a means to check fuel pressure without it being parked where I can plug in my gauge. With a very lean exhaust I am curious about fuel supply though...
Well as you said the diagnosing continues, Ill keep you guys informed as it unravels.
 
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Discussion Starter · #9 ·
Hey guys,
So update for the Jeep Grand Cherokee....The Coolant temp sensor was replaced and now the vehicle doesn't have the hesitation anymore. My guess is the sensor was failing and giving inaccurate readings to the ECM which made it switch constantly between open and closed loop which gave the hesitation and sputtering. Now I'm back to the last few maintenance items on it and it should be good to go.
I thank you both again for your help and wisdom.
 
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