sabre85rdj

I'm stumped so wondering if someone can give me a suggestion. I posted this on the jeep forum but I've not gotten any suggestion. I bought a 2001 Cherokee with a 4.7 V8 engine to fix up for my sister. It's got high mileage but the car is in good shape. The engine cranks just fine and the engine tries to fire but it won't start. I've checked the spark on no. 1 cylinder and it has a good spark. I've poured a small amount of fuel into the throttle body but it doesn't fire. If I hold the throttle wide open it helps but it backfires into the intake manifold like the ignition is out of time. I've checked compression on all of the cylinders and the compression varies some from cylinder to cylinder but it should be good enough to run 120-150 psi. I think the valves are leaking a bit because when I add oil to the cylinder, the compression doesn't improve much. The car has been sitting for 4 months so I wouldn't be surprised if there wasn't a little rust on some of the valve seats. I can't put a timing light on the ignition to check the timing. As I understand the manual, the crank position sensor and the PCM control the timing and the cam sensor controls the fuel but since there's spark, I didn't think it's likely to be a crank sensor. The ignition seems to be somewhat random since when it fires, it kicks back against the starter and stalls the engine during cranking. Does this sound like anything familiar?

Bustedback

Load test the battery. I know the engine cranks, but trust me, load test the battery.

sabre85rdj

Well, I got the problem fixed and it turned out to be the crank sensor. It's a PITB to get to but once I replaced it and the engine started and ran perfectly. I paid an extra $10 to get the OEM part from the dealer based on comments I've read about aftermarket parts be questionable. Given the trouble I went through to get it out, I didn't want to have to change it out due to a poor quality part. I had no idea a sensor could fail in a way to give erroneous signals to the ECM to fire the plugs at the wrong timing. Usually, a sensor works or it doesn't.

Dimewylde69

​​​​mines doing the exact same thing but I changed the timing chain crank and cam position sensor, still no start

Cleman Simpson

You don't have a gas tank full of water do you? sounds stupid but I've seen it more than once.

edit to add. Everybody knows you have to have fuel, compression, spark, and correct ignition and injector timing along with sufficient starter rotation speed to get an engine to crank and run. If all these seem to be present and the engine still won't start then you are getting a false positive on one or more of the required parameters. Shooting some starting fluid down the throttle body will usually tell you if you are dealing with bad fuel and examining the plugs will give you a hint as to whether the ignition spark is just too weak to fire the engine when it is cold. With computer controlled timing the ecm would be the primary suspect if all other basic problems have been positively ruled out. I hate to beat a dead horse but once again that small lead off the positive battery cable can and will deliver low voltage to the ignition system which will cause the coil pack to not generate the required voltage to make a good spark under compression, as will a bad engine ground also.

Dimewylde69

would that cause to not start even with starting fluid? I spray it in and doesn't fire up at all, even for just a second

Cleman Simpson

Usually even if you have a bunch of water in your tank and fuel lines starting fluid will overcome it momentarily and allow the engine to fire.

Dimewylde69

yea, that's what I would think, I'm at the point now that I'm just gonna put it in the shop once I have the cash

Cleman Simpson

Have you pulled out some of the plugs to see if they are wet after trying to start it? If they are then you probably have weak ignition spark.

dave1123

Yeah but! Weak primary voltage can cause all sorts of weird things to happen. Check all your grounds. A visible spark may not have the voltage to fire the plugs under compression pressure.

Case in point. My 4.0 was burning oil and fouling the plug in #5 cylinder only at idle and the compression was good. A new coil rail solved the entire problem.

Raymondandcallie Bertrand

it got hot I pulled over let it cool down, needed a jump to get it started once it started it ran poorly. So I shut it off and towed it home and hasn’t started yet
2005 grand Cherokee laradeo 4.7