ZJ hard starting/drop in mileage

Rox&Mud

Rox&Mud

CTB
Joined
Nov 3, 2006
Location
Raleigh
I figured I see if any of you other 4.0 Jeep guys have ever had a problem like this. It seems that, rather suddenly, my ZJ became hard to start, meaning having to run the starter motor for much longer than the normal amount of time. It has always started eventually and occasionally will fire right up. When it starts after a long attempt it usually smells as if it had been flooded. Since this began, my fuel mileage has suffered by about 25% on average. The only known problem is the common 4.0 exhaust break at the collector, but I don't see how this could cause the other problem. I swapped in a new cap and rotor with no change. Checked plugs and they all looked good. Wires, plugs, cap, rotor and coil were changed about 10K miles ago.
Any insight would be helpful.
 
bad o-rings on an injector or the fuel pressure regulator? time for a fuel filter maybe? DOn't forget vacum leaks, one of the hard plastic lines may have developed a crack due to vibration and age.
 
Vacuum? It may very well be the problem or part of it anyway. I have noticed that when going uphill with the cruise and AC/heat on, in other than defrost position, the AC/heat will default to defrost and the ruise has a hard time keeping speed. The fuel filter I can check/change but what about the o-rings, how hard are they to check/change?
 
Rox&Mud said:
Vacuum? It may very well be the problem or part of it anyway. I have noticed that when going uphill with the cruise and AC/heat on, in other than defrost position, the AC/heat will default to defrost and the ruise has a hard time keeping speed. The fuel filter I can check/change but what about the o-rings, how hard are they to check/change?
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my '98 gr cherokee had the exact same problem with the hvac defaulting to defrost going up hills. my vacuum leak turned out to be a disconnected line @ the vacuum reservoir. they tend to dryrott. the vacuum reservoir is located under your battery tray.
 
jeepman13 said:
my '98 gr cherokee had the exact same problem with the hvac defaulting to defrost going up hills. my vacuum leak turned out to be a disconnected line @ the vacuum reservoir. they tend to dryrott. the vacuum reservoir is located under your battery tray.
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Think I found my vacum leak, mine was at a dry rotted connector at the evap cannister. I won't know for sure until I finish going over a few other things looking for the starting/mileage problem.
 
Well the dry rotted vacuum connector did not fix either problem. Still hard starts and the ac/heat still goes to default when using the cruise and going up hill. I addressed the exhaust leak over the weekend by replacing the broken manifold. I also replaced the injector o rings in the process. I don't expect those to correct the problem but we will see.

Well, I've had a chance to drive it a little and it's still having all the same problems.
 
I have seen this in the past,possible one or more fuel injectors leaking down after engine has been shut off.Leaking fuel down into cylinder and causing extended crank time,then once started smell the raw fuel.A quick check would be hook up a fuel pressure gauge to rail,start engine let idle a minute or two,shut off engine,watch gauge,should hold pressure for a while,if pressure drops off possible injector leak,or you could start engine then shut off,remove fuel rail assembly place a piece of cardboard under all the injectors and let set a while should be no fuel leaking from injectors.As for the vacuum leak check to make sure that the speed control servo itself is not leaking.I hope this will help out if not let me know,have more info. if needed.
 
man i figured that vacuum leak would take care of it. this may be a stupid question but is your air filter ok?i have seen this cause a similiar problem on a dodge van.
 
jeepman13 said:
man i figured that vacuum leak would take care of it. this may be a stupid question but is your air filter ok?i have seen this cause a similiar problem on a dodge van.
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Actually, I just added a custom cold air intake so I had room to put the evap cannister under the hood. The problem was there before and after that little upgrade.

Still haven't had a chance to check try njec 4x4's suggestion.
 
but it does it went has not sat for a while ie go into gas station come right back out------------hard start---------then let it sit 6 hours no hard start. or vise versa its random.
 
I dont think you posted what year your grand is, but it sounds like when my crankshaft position sensor went out. The jeep was real hard starting, but ran fine once running and gas milage went to crap. Also if your jeep is OBD 1 it could be the oxygen sensor. I have seen OBD1 cars have the same simtoms when the O2 sensor went out.
 
It's a '98 ZJ so it should be OBD II.

...and as Chase said it is very random. I can start it after three days and it will fire right up then continue to start properly the next few times and then return to hard starting again or vice versa. It is an irritating problem especially since it is so intermittent and not throwing a code.
 
If you push on the gas a little will it start right up?
 
Ive checked the crank signal during the no start condition and it is good. My only thought that i haven't had time to check is if the fuel pump is either not pumping or not getting fire randomly. I have seen it on hondas with their EFI relays going in and out.
 
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