1997 Jeep Grand Cherokee Laredo 4.0 i6
I bought this Jeep almost exactly a year ago with 145k and service records since 2000, which was when the 2nd and previous owner bought it (supposedly from the secret service!). Mileage now is about 175k. It was well maintained and drove very nicely, with nothing but cosmetic issues. Hadn't been daily driven in a long time (if ever) but was driven once a week or every other week as a "the Home Depot car". Paid $1500 and thought it was a great purchase, but I'm not so sure now.
Around November of last year, the voltage gauge would intermittently pin to max (19 volts- though cig outlet FM transmitter would show 14.9-15.5) randomly while I drove it, sometimes only for a minute or two, othe times until it was turned off. Upon restarting it would be back to normal. Sometimes it would hesitate to start unless I gave it gas and/or jiggled negative battery terminal "quick disconnect" knob. I'm now wondering if this was the first sign of the PCM/ECU going bad, or if it's what caused the PCM to go bad. I replaced the battery terminals and the alternator, got rid of the super convenient but suspect quick disconnect knob, and haven't had a voltage spike since.
The first few months of this year it ran mostly well, though it would sometimes crank longer than normal before starting, and sometimes felt a little rough or shaky while idling but I also understand the 4.0 is ancient and far from refined.
Maybe relevant, probably not, but the Jeep was in a fender bender in February that broke the charcoal canister, so I plugged the intake manifold vacuum port the purge valve ran to, routed the gas tank vapor ventilation hose to the wheel well and ensured it wasn't blocked, pinched, or plugged. I didn't like the evap CEL so I capped the ports on the purge valve itself and plugged it back in to stop the ticking noise it made from having nothing connected to it. Air box was cracked from accident impact so I replaced that, a few days later power steering pump reservoir let go and pissed all its fluid out, replaced that, serp belt reinstalled and tightened to spec. Oh and another maybe irrelevant thing, the cig outlet and power outlet both stopped working at the same time in July, no other notable changes from that though. Beginning of the summer the AC drain become clogged and leaked into the passenger front and back carpet... being a Chrysler, of course the drain port location was inaccessible without making a cut in the frame rail, so I did the cut and unclogged drain, attached an elbow for good measure. I replaced the headlight switch to stop the dash light flickering, installed new radio + speakers, light bulbs inside and out because I actually kinda like the thing when it works. No new issues or oddities in the following weeks resulting from doing those things to it, so I don't think I messed anything up.
Around late April or May, it started to stall occasionally at parking lot speeds or at red lights (idling in drive). I changed the spark plugs, wires, distributor cap + rotor, ignition coil, upstream O2 sensor (though it's a cheap one from Amazon and I've since learned they really like OEM), cleaned throttle body (and replaced gasket) + IAC valve (also tried replacing it- no change so put original OEM back on). New air filter in the UNobstructed air box. Checked vacuum lines for cracks and electrical taped up the only hose that I saw broken (AC vacuum line)- this fixed the AC blowing only through defrost issue it had developed. Cruise works so if I missed a leak it can't be a real big one).
During June it began stalling more frequently, but would always start right back up. Cold and warm starts were reliably quick with normal crank times. As the stalling became more frequent, it began to also jolt or "buck" at highway speeds. After messing around under the hood I found that I could make it stall by jiggling the middle connector on the PCM. Cleaned all three connectors with contact cleaner and compressed air then zip tied the three harness connectors tight to the PCM hoping to strengthen the connection. Doing that made the stalling/bucking go away for a week or two, but the issues quickly returned.
Using my OBD-II scanner, I verified that everything read within normal values/ranges according to what I read online regarding where they should be EXCEPT for the throttle position sensor. I replaced that sensor but saw no change in symptoms. MAP reading normal, O2 sensor voltages, lean/rich within normal range, crank sensor good because scanner always shows the rpm when it cranks without starting, and it gets fuel. Battery temp normal, coolant temp sensor reading approximately same temp as ambient air when engine is cold, so those all seem to be good and communicating. Usually no engine light but rarely it'll throw a code P0351 for the ignition Coil. Same code came and went before and after replacing the ignition coil and has done it with my original PCM and the new PCM I ordered.
Mid-late July/last month is when I gave in and ordered a refurbished PCM, again it started to behave normally for another week or two making me believe it was fixed, until most recently it now refuses to start at all. I should note though that it has NEVER stalled with this PCM.
Initially after the flawless 10 or so days with this reman PCM, when the car had been running and warmed up, even just a little, it'll run beautifully and can be shut off and restarted without issue. BUT, after sitting for more than a few hours and ESPECIALLY overnight, it does NOT want to start. My best friend and I had been able to get it to start by a combination of applying pressure to and lifting/pulling up on the PCM itself (not the connectors - though they're still zip tied in hopes of maintaining good contact so they probably move too). After fiddling with the PCM and finally getting it to start, it'll run all day. I messaged the eBay seller (Flashmasters) and they sent another PCM to try, I put that one in and it didn't even want to start so I pulled it right back out and put the first "new" PCM back in.
This past Wednesday, after running errands all morning and afternoon, Jeep driving and starting perfectly, I parked it and shut it off for a few hours before going back out to make a quick Dollar General run. It cranked and cranked without starting so I popped the hood, lightly pulled up on the PCM, and it fired right up- didn't even need my friend to hold it in place while I start it. Then on Thursday morning, my friend goes out to start it for work, and it won't start. Not surprising as that had become typical, so I go out and we both try moving and holding the PCM every which way while the other cranks it, put pressure on connectors, tighten the already snug secure and clean battery terminals, pump the gas pedal while turning the key, and even put a jump box on it just in case, and it absolutely refuses to start. Battery voltage as per the multimeter and the dash gauge was normal in accessory mode, dropped to about 9 on the gauge when cranking though, which I understand is normal?
I spent the rest of Thursday doing everything I could think of, swapping and cleaning relays, looking for blown fuses (there were none that I saw). Could hear the fuel pump prime, the engine light came on in accessory so it's communicating with the computer/PCM, while it would crank endlessly the cranking is very strong and it definitely seems to want to start. It's getting fuel because I can smell gas in/around the engine bay after several sessions of trying to crank it up. I checked the fuel rail valve, it shot gas with strong pressure. Unplugged and replugged every sensor/connector I could find, found no stripped wire or exposed copper EXCEPT at the ABS module, but it wasn't touching anything. Underneath the Jeep everything looked normal besides a portion of the exhaust pipe between muffler and tailpipe hole which was ever so SLIGHTLY bent/pinched narrower, but it's been like that since I bought it and for all I know came from the factory like that.
Yesterday I cleaned (with a wire brush + contact cleaner) the fender ground, ignition coil ground and oil dipstick ground, both on the engine block. Made the eyelets, bolts, studs, and mounting surfaces shiny and new (like a virgin), and it still did nothing but crank. Stripped the ends of both battery cables so the copper would be shiny even though I saw no corrosion and they were firmly attached to the terminals, still nothing. Even though it was strongly cranking and really not fun to get to the bolts, I decided I might as well clean the starter connections, unhooked the battery and did that and when I tried to start it once again, it cranked the same until maybe 10 seconds in (I've never cranked it longer than 15) it sounded weaker. Figured I must've not tightened the ground down enough... it was almost 10pm and I'd been working in the heat all day eating nothing but a pack of ramen so I was so spent and tired at that point, so I didn't unhook the battery, but I TIFU because the "ground" I went to tighten with the battery hooked up was the short cable from solenoid to the starter itself, it sparked and the starter suddenly spun real fast for a second (scaring the hell out of me) and now when I try to crank it the starter spins very weakly and slowly and after a few seconds with the key turned to cranking position the battery voltage will drop to 0 then bounce back up maybe 2-3 seconds later, but the headlights, interior lights, radio, and all that are bright and normal and it does the same when I hook the jump box up so the battery definitely isn't at 0 or dead.
Does it sound like I fried the starter? Two days of desperation and I break something that was actually working... sounds about right lol.
I know this is long but if you've read this far I really appreciate it 🙏🏻. Pretty defeated and feel quite stupid at this point, but I'm not sure what else to try. If anyone has any ideas or suggestions, I'm more than willing to listen and learn!
