Another no start -- injectors not firing.

Hi guys,

My 94 4.0/AX-15 died yesterday and I have been unable to track down the cause.

I was driving home after a few hours wheeling and when I tried to accelerate quicky from about 45mph, the engine missed/backfired a little. I left off the gas resumed cruising and then tried again, same thing. Then the Jeep started bucking/missing really hard with any load on it. Had to slow down and pull of the side of road where it then died.

I started to diagnose things and here is what I have.

I have spark. At first I thought the fuel pump was intermittent and would work after I hit the tank a few times with a hammer. Now it seems fine and I have good pressure to the rail. The Jeep starts right up with some starting fluid in the intake.

I checked the injectors and it looks they aren't getting any signal to open while cranking.

What sensor controls the injectors? I replaced the crank position sensor about a month ago. From what I understand though, if the CPS was out, there would be no spark as well. I checked the plug at the ECU and it was clean. The ground wire connections all look good, too.

At this point I am thinking its the camshaft position sensor inside the dizzy. Considering throwing in a rebuilt dizzy, but would like to narrow it down a little more before I put any money in the heep. :wierd:


Thanks for any help guys.
Chad

Synch Signal Sensor-Stator, aka camshaft position sensor sounds like a candidate for the source of your problem. In the distributor.

Cool. I was starting to think the same thing and its good to hear someone else confirm the idea.

I will test it tomorrow, or maybe just replace it and see what happens. Hopefully it's something simple like that. :looney:

At least try working the connector (unplug/plug a few times) to the CMP (cam position sensor) before replacing it. More often then not, the problem is corroded connectors. (with any electrical component).

I've had cam position sensors fail on a 93 and a 95. In both cases, they threw a code. Also, in both cases, I was able to start the engine by first turning on the ignition, and then, with ignition still on, unplugging and replugging the distributor plug. You get one start per operation, and the engine will not run terribly well. since the injector pulses are not timed properly (my 95 ran hot) but it should get you home if you're patient.

On the 93, the sensor replacement required the distributor to come off and apart, but on the 95, it was a simple replacement in place. Neither was terribly complicated, but of course on the 93 I had to be careful putting the distributor back in. I don't know which type the 94 uses, but I would certainly not opt for a rebuilt distributor if it's the latter type, because it's as simple as can be.

Mine has the easy one. It looks like a disc with a sensor on it and it just lifts out of the dizzy.

I checked for codes, but only had one for disconnecting the battery, which I did about 2 weeks ago.

I will try checking and cleaning the plug and then the disconnect/reconnect trick to see if it works.

The Jeep was running pretty hot most of the day (it was probably 90* out and I was moving slow on the trails too). It also seemed to have some power issues and was pinging frequently. Up until now, I have been dealing with intermittent power loss (seems to come and go and varies with load, feels like a fuel delivery problem) and some pinging at low rpm and high load. Over the course of two years, I have replaced the coolant sensor, CPS, O2 sensor, cap, rotor, wires, plugs, and TPS. All helped to increase gas mileage and smooth it out a bit, but none solved the problem.

I would be estatic to replace this sensor and have it run great afterwards. I test drove a 99 automatic and was impressed by the power and how smooth the engine was. Night and day compared to mine. It was more like the 4.0 that everyone describes it as.

I found the problem! arty:

After testing the Camshaft Postion Sensor, I found that it was delivering a good signal to the ECU. Then we started diagnosing the wiring for the injectors and confirmed that there was no power to the injectors.

We confirmed power on the same circuit was indeed going from the ASD relay to the ECU, so we just had to find out why it wasn't getting to the injectors. It ended up being the factory splice where the injector power wires all meet up and get their power. It's located right in the bend leaving the plastic wiring channel that runs along the valve cover and houses the wiring for the O2 sensor, injectors, and coolant temp sensor.


Thanks for all the help guys! Keeping the ol' heep running is certainly a group effort these days! :laugh:

american_zero said:

I found the problem! arty:

After testing the Camshaft Postion Sensor, I found that it was delivering a good signal to the ECU. Then we started diagnosing the wiring for the injectors and confirmed that there was no power to the injectors.

We confirmed power on the same circuit was indeed going from the ASD relay to the ECU, so we just had to find out why it wasn't getting to the injectors. It ended up being the factory splice where the injector power wires all meet up and get their power. It's located right in the bend leaving the plastic wiring channel that runs along the valve cover and houses the wiring for the O2 sensor, injectors, and coolant temp sensor.


Thanks for all the help guys! Keeping the ol' heep running is certainly a group effort these days! :laugh:

Click to expand...

Good troubleshooting. I had a vexing splice problem on the 93, but it was further up the line, and only killed one injector, and only when it heated up. It hadn't occurred to me that the whole kaboodle would go out at once.

Took it for a 4 hour drive for the 4th and not only does it run better than before, I managed 22mpg for the trip. Quite impressive for having stock gears, 30" tires, and a 3" lift, if you ask me. Actually the best mileage I have gotten out of it, even when it was stock.

Matt -- It definitely takes some patience to troubleshoot like that, but you just have to be careful that you are keeping track of what you have checked already and do it methodically. We confused ourselves a couple of times by looking at the wrong wiring diagram and switching the voltmeter to ohms and back to volts. Had to slow down a bit to keep it all straight.

Previously, my Jeep had a problem making good power under load, but it was intermittent. It runs better now and the only thing I can think is that under load the engine movement was cutting power to one or more injectors. It didn't happen all the time, and by easing up on the throttle and getting back on it, it would sometimes start to run better. If you rolled into the throttle rather than slamming it wide open, it would sometimes avoid the problem as well. It was very annoying and I replaced numerous sensors trying to get it to run right with little success. Perhaps this was the key.