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99 dies at high rpm after manifold replacement


NAXJA Forum User
San Francisco
It's a long story but I'll try to keep it to the need-to-know facts.

I replaced my exhaust manifold and gasket with an aftermarket one last week. Had a bit of trouble but eventually with a new flange donut I got it all sorted it out.

I took it for a drive through the hills and it performed great, lots of power, no noises, etc. Really feeling good about it. I probably drove 50-60 miles, stopped a couple of times to check out a view or grab food. When I was headed back to the city I was cruising on the flats at about 45mph and all of a sudden it lurched on me, continued on but at low power and then quickly died. Just stopped running. I pulled over and tried to turn it over but it wouldn't even crank. All the electrical was good but couldn't get the engine to even crank.

I popped the hood and noticed that I'm a giant jackass and had forgotten to attach the front CCV hose to the airbox. I figured that's not the biggest problem because that just means it's sucking unfiltered air and we didn't drive through dirt or anything.

I let the engine cool down for maybe 15min and then started it up. It started right up and had what felt like full power but I was certainly not trying to test it. The problem is that to get back to the city I have to climb a gigantic hill and then cross the golden gate bridge. While climbing the last big hill I felt it lurch again and lose power. I pulled off on the last exit. The engine didn't die but I sat there for a few minutes anyway. Pulled back out, got on the Golden Gate Bridge and it promptly died. I ended up getting out and pushing my jeep across. I'm surely in a lot of tourists instagram posts.

Once again, we let it rest for 20-30min, started it up and drove home about 5 miles.

Has anyone experienced these types of problems? What puzzles me is that it won't start up again until it's cooled down. If it died because of a vacuum problem I don't know what would stop it from cranking. But I also don't know anything about engines.

I'm already going to check the ignition coil but that's almost hopeful. It's almost guaranteed to be a problem with the vacuum or exhaust system. Any suggestions are much appreciated.
Failing nss or wiring in the column are the only culprits i can think of as far as the no crank condition. The hot running issue can be any number of things, tps, cps, cam sensor, even fuel pump or relay. Next time it doesn't want to crank try nudging the shifter fore and aft as you hold the key in start.
When my NSS gets dirty I can usually start it in Neutral instead of Park if the wiggle trick doesn't work.

It sounds like you have a heat related problem. What exhaust manifold did you fit? Did you wrap it with heat wrap? Heat wrap on exhaust manifolds can increase heat.

As it's a 1999 you should be able to read any stored diagnostic codes. A stored code can really help track down the root cause of a problem. I prefer the Bluetooth style of code reader that uses a smartphone app.
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This it's what mine did with the could died... Not saying that's your issue, but it was definitely mine.

Run the Jeep until it dies, hop out and do some diagnostics... Check to see if you hear the pump prime, check for spark, get a mood light and check for injector pulse... This is the first step

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I've had a similar issue when the CPS failed. Thermal failure with intermittent restarts, then it lets go completely.
..... struggling to find a good answer to why it would die and then not crank until cool.

This is a primary symptom of a failing Crankshaft Position Sensor, but may be any other sensor or the fuel pump. The CPS can be tested, but may only test out of spec when warm. Heat related failure of the CPS is common. A faulty CPS does not typically generate any CEL trouble codes.

Buy sure to only install genuine Jeep engine sensors and not any cheap Chinesium junk.
Don't assume and clean the battery cable clamps and lugs on the battery. Shake the ground cable where it connects to the block. I clean my battery clamps and lugs with solvent, then with mild sandpaper, and then wipe both down with solvent again. Really too basic of a thing to trust to luck.

Ask yourself if the problems are always on a hot engine? The O2 sensor will usually kick in after a few minutes of driving and if it is flaky the engine can buck or outright quit. O2 CEL often doesn't present until after the O2 has completely crapped out.

Is it possible your O2 wiring is cooking on your new manifold?
Is it possible you damaged the O2 sensor?
Is the O2 sensor connector oil soaked?
The O2 sensor would not have anything to do with a no crank, but is it possible you may have multiple issues?