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Engine rotates but will not start.

2K views 30 replies 11 participants last post by  pitsy 
#1 · (Edited)
Here's what I know: Battery is good. Fuel pump works, fuel level sender works. Fuel pump relay and ASD relay are good. All fuses in power distribution center are good; all fuses under the dash are good. There is fuel at the rail, but I have not gauged the pressure yet.

I disconnected the ignition coil rail connector and checked for voltage between the DG/OR wire and ground with the key to 'ON'... Zero Volts. Should show system voltage, right?

Also, the instrument cluster illuminates, but doesn't read anything on any of the gauges.

What do I check next? I feel like I'm missing something obvious.

This Jeep was driven up north and has 131k miles on it. Wiring in the engine looks clean, but everything underneath this beast is sketchy.

This is my first Jeep, given to me by a family member. It was running a few months ago but it wouldn't start one day so there it sits. Help me get it running so I can drive it!
 
#2 ·
You clean the battery terminal heads? Similar thing happened to me and it was due to corrosion on the neg battery terminal. Cleaned it up and it started back up
 
#3 ·
Also jump start the car using another vehicle. Many times the battery "tests" good, but doesn't have the juice to start the Jeep. If it starts up, it's a bad battery.
 
#4 ·
You say the engine rotates so I'm going to assume that its turning over strongly.

No idea about the voltage at the connection you are asking about, but start with the absolute basics.

For an engine to run you need fuel, spark and compression. If it just stopped running one day without a major incident then compression is likely fine.

Give yourself the best chance you can and get fresh fuel in there. Turn it over for a bit trying to start it and then pull a spark plug. Is it wet with fuel?

If not, you've got a fuel issue.

If so, then its a pretty safe bet its a spark issue.

Does it stumble and try to start at all? Or just turns over and over and over?
 
#5 ·
OK, more info. The initial problem was a burnt fuel pump, which I've replaced with a new unit. Before there was no fuel pressure at the rail and the pump wouldn't run. After replacing it, the pump runs and there is pressure.

The engine turns over strong, battery terminals are clean. Five gallons of fresh fuel in a clean tank.

The Haynes manual says I should see battery voltage at the wire I mentioned, as it supplies power to the coil rail.

I have until next Tuesday to come up with something to test or replace. Been racking my brains and trying to read the schematic. I SUCK at reading schematics.
 
#7 ·
I have not checked the grounds yet, but I will. However, shouldn't there still be voltage present at the coil rail? I tried with the negative multimeter probe on the block, a body ground, and the battery negative terminal, and the positive lead on the feed wire in the connector.. Zero voltage.
 
#9 · (Edited)
No, that's helpful! I feel like it's something obvious, so checking grounds is good advice.

Any idea why the gauges aren't reading? They should at least show battery voltage and fuel level with the key on....
 
#10 ·
That's why I'm suggesting grounds-others have reported the same symptoms. Also, who checked the battery? Just because the voltage is good doesn't mean it has enough power to satify the PCM and start the jeep. Try to jump it with another vehicle. It has worked for others.
 
#13 ·
There's no follow-up yet. Sunday is family day and I prob don't have the gas to get out there Tuesday. In the meantime I'm in research mode, learning about the systems involved and formulating hypotheses. There's no real pressure to get it running, other than my GF's dad wanting it off his driveway, and the fact that I enjoy a challenge. I've got an hour free tonight; I'm gonna stare at the wiring diagram and see if something in my brain clicks.
 
#14 ·
Go back to the basics.... get a can of starting fluid aerosol (2 is best) and have someone spray the intake (Airbox plumbing off) while you turn it over. That will tell you where the problem is and is not. If it runs, the problem is fuel delivery or bad fuel. If not, the problem is likely after fuel delivery such as electircal, PCM or ignition-related.
 
#15 ·
If you do the key thing to check for codes and get no response there is something wring with the PCM or its communication network.

When you say the instrument cluster gauges are dead do you have anything showing in the odometer display. That cluster is a computer in and of itself. It receives all its info from the network (PCI Buss) that it uses to communicate with all the modules.
 
#16 ·
PCM is dead, get a rebuilt one and have it programmed to your VIN. No cheap way of getting out of this one I'm afraid, I had the same problem. Use a meter to check your TPS for any juice, if it has nothing, your PCM is definitely dead.
 
#19 ·
Not entirely.

You can get a used ECM from a junkyard and have it flashed to your VIN. I recently did this with a WJ I bought for cheap due to a no start condition. I paid $35 for the ECM and $65 for the flash at a local parts store.

Worst case scenario take it to a local dealer for the flash. All my local ones quoted me $100 to do it.
 
#17 ·
How long has it been sitting? You say it ran fine, and then sat for a while?

If the battery has enough charge to turn over the engine, then I don't suspect it's a voltage supply problem. And I'm not sure, correct me if I'm wrong, but the starter shares a common grounding bond with the engine block. So it the starter turns, the grounds are OK to the block.

I'm still concerned about no gauge activity. Jer-Bear and 86cj74.2L might be on the right track with the PCM. Or the voltage supply to the PCM. Fuses? Oxidized connector pins?

Keep the 'updates' coming. Between us all, we'll get to the bottom of it.


EDIT - Just checked the FSM for my 99 4.0. The Automatic Shutdown Relay supples voltage to the ignition coils. (The OP said he had no voltage there.) The ground circuit to operate the ASD relay is controlled by the PCM. The PCM regulates ASD relay operations by switching its ground circuit on-and-off.

Prior to looking at the PCM, you can check Fuse 6 (30A) in the Power Distribution Center. It supplies voltage to the ASD relay. You can also check the relay which is also in the PDC. The switched relay contacts then feed voltage to Fuse 26 (15A) also in the PDC. This fuse supplies voltage (finally) to the PCM. All easy stuff to check with your volt meter.
 
#18 ·
Minor update: Confirmed the issue is electrical by spraying starter fluid into the TB while cranking. Battery voltage was 11.6V, so I pulled it and had it load tested. Bad battery. New battery installed turns it over better, but still no start.

BUT, there is full system voltage at the coil rail connector. Using my spanking-new FSM, I was able to check every fuse and relay in the starting/ignition circuits. I also jumpered system voltage into the ASD relay output slot in the PDC and measured no voltage drop between there and the coil rail connector.

I've disconnected and cleaned all the harness connectors on that side, and everything hooked up to the TB, plus every connector going into the junction box on the firewall. Still no start.

I'm finally having it towed to my place so I'll be able to work on it more comfortably. I'm about to get medieval on that wiring harness.

Oh, to answer the question about the instrument cluster: When I turn the key to ON, the odometer displays mileage and various warning lights illuminate briefly. No readings on the fuel gauge, battery gauge, or the other needle-type gauges.
 
#20 ·
Oh, to answer the question about the instrument cluster: When I turn the key to ON, the odometer displays mileage and various warning lights illuminate briefly. No readings on the fuel gauge, battery gauge, or the other needle-type gauges.
Sorry posting via mobile so no multi-quote.

The no readings from the fuel and battery gauges typically means the ECM is not powering up. Like Jer-bear mentioned you can check for 5V power at the TPS with the key on to determine if the ECM is functioning.

If the ECM is dead, refer to my earlier post.
 
#26 ·
Thanks for the help everyone. I do have a good source for a used ECM, if it comes to that. But like I mentioned, there's a lot of corrosion going on so I'm gonna subject the wiring harness to some serious scrutiny. I'm really excited about having it towed to my place tomorrow! It'll be right outside my back door, with an autozone on the next block.
 
#29 ·
OK, it's been delivered to my place, safe and sound. I've got a flat, clean workspace (outdoors), electrical outlets for tools and lights, and a fridge and internet twenty feet away. If I have time tomorrow, I'll take a good set of "before" pictures inside and out to document this project. I'll continue this thread until I get the thing running, but I'll start a new thread for the overall project.

The theme for this project is "Shoestring, Junk-Yard, Handmade". The sub-theme is "When was your last tetanus shot?".
 
#31 ·
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