• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Intermittent Fuel Injector Pulse

tempest411

NAXJA Forum User
Hello,


My '88 XJ has been giving me fits since late last summer. Initially it would stall while driving down the road, but would start back up right away. Changing the CPS did nothing to effect that. Then I noticed that when the engine stalls, as inertia is still spinning the engine over I had a tach signal, so I thought I therefore must have ignition, so the fuel injectors must not be firing. As the CPS was new, and wired straight to the ECU 20 years ago when I first got it, I suspected the computer. I suspected the fuel injector driver circuit must be on the way out. Well...two other known good ECUs didn't fix that:/ With a noid light on an injector connector I get a bright flash only on what I suspect is TDC #1. I didn't actually verify this, but the flashing is very consistent with a crank position it seems...and you can here the engine fire briefly when the noid light flashes.



What other failures might cause this problem? Even with a new Mopar CPS, it's output was always weak relative to the ideal quoted everywhere. I'd always heard it should be .5V or better, but the most I'd ever gotten was .25V. It ran for 20 years that way. I did pull the CPS out and drill out one of the holes to 3/8" and re-fit it. It did increase the output to .31V, but made no difference it now being stuck in front of my house...I started it a few weeks ago and it ran fine, but this week I can't get it to start at all.



Any thoughts? I'm tempted to start looking at the distributor, though every source I've come across says a failed pickup can cause driveability problems, but never a no-start.


Thank you for your advice!
 
First thing you should do is visit cruiser54.com and follow his tips on refreshing the ground connections and cleaning or eliminating the C101 connector. Also have a good look at the wiring harness from the injectors and make sure it's not damaged or rubbing at the back of the valve cover.
 
While grounding is important overall, it has nothing to do with the injectors. The injectors are fed by a constant (ign on) B+ supply and their individual grounds are controlled (switched) by the PCM, so if you had a grounding issue with the PCM you would have a lot more issues!
 
Thank you for the replies. Last night after dark I decided to look at the spark output, just for the heck of it. It seemed intermittent as well! Makes me think I've got a problem with power or ground to the ECU. I will investigate further...after my daughter's birthday.
 
While grounding is important overall, it has nothing to do with the injectors. The injectors are fed by a constant (ign on) B+ supply and their individual grounds are controlled (switched) by the PCM, so if you had a grounding issue with the PCM you would have a lot more issues!

Renix are fired on the plus and all share a common ground, along with all the sensors on the engine harness. You are thinking of 91 and later. That ground in the engine harness is daisy-chained through a series of splices. There is alot that can go wrong in that harness.

I found one of those splices in a crimp connector that was never crimped. I also found once a wire core that fatigued through *under the insulation* from flex and vibration fatigue, at the firewall bend in this harness. That one was fun to find.
 
Renix are fired on the plus and all share a common ground, along with all the sensors on the engine harness. You are thinking of 91 and later. That ground in the engine harness is daisy-chained through a series of splices. There is alot that can go wrong in that harness.

I found one of those splices in a crimp connector that was never crimped. I also found once a wire core that fatigued through *under the insulation* from flex and vibration fatigue, at the firewall bend in this harness. That one was fun to find.


My 89 had a crimp connection in the wiring harness on the passenger side that was just a crimp with no butt connector around it. The ground connection by the dipstick tube is a problem area, as is the connections under the coil to the ignition module. The Cruiser54 Renix tips covers them all, and they're a great place to start for intermittent electrical issues.
 
I have cleaned several of my grounds, including the one by the dipstick tube, as in disassemble and wire brush everything and put it back together. I also replaced the 'C-101' connector with a couple Deutsch-Tech connectors.
 
I have cleaned several of my grounds, including the one by the dipstick tube, as in disassemble and wire brush everything and put it back together. I also replaced the 'C-101' connector with a couple Deutsch-Tech connectors.




You're quick. Any improvement? I'm skeptical that it's the engine computer, as usually the drivers just outright die and not become intermittent. I think it's more likely a wiring issue.
 
Check the harness for chafing near the head bolt that has the braided ground cable attached to it. Done Tip 5?


I've spent a few hours on this the last couple of days. In so doing stuff like this I always end up reverse engineering things in order to understand how it works, which can lead to lots of rabbit holes. In as short and concise a fashion as I can be, here's what I've found. At the ECU I have good battery voltage, good switched voltage from the ignition switch, and good voltage from the B+ relay (10.4V, which is good from what I understand). I also ran a resistance check on the two ground leads at the ECU, pins B11 and B12 back to the battery negative post and found a had 13 Ohms of resistance. For a connection you'd like to have 0 Ohms on, that seems high to me. For comparison I did the same check between the ground terminal on the D1 diagnostic connector under the hood and the battery negative post and got .4 Ohms, which is good. According to the schematic, it's supposed to be an uninterrupted pathway. No connectors are shown. Could there be perhaps a welded splice somewhere that's gone bad through oxidation?



On kind of a whim, I shot some starting spray down the intake to if the ignition system was alive, and it is. It will run for several seconds and die. In 95 degree heat starting spray sure doesn't hang around very long though! I also discovered something about the injection system that surprises me; they are not ground-switched as I had assumed. One side of the injector connectors goes straight to ground, while the other side sees a pulsed +12V from the ECU. And because the ECU has independent leads for each injector, are these vehicles actually sequentially? I was really expecting these to be a batch-fired design. Sequential port is pretty fancy for these things. Not even my '87 Porsche has that...One thought that just occurred to me is that I forgot to check EVERY injector connector. I've only checked #1 for pulse, and coincidentally when it would pulse I could hear the engine start to kick over, which means IF it's sequentially pulsed, the injector that fired right behind it is working, taking me back to a injector wiring problem...I've got no visibly damaged wires at the rear of the valve cover. There are no wires even close back there. Oh! Yes, I have 31 psi of fuel pressure while cranking...



Lastly, I decided to check the coolant temp sensor to see if it was both intact and reading accurately. I don't know if it's relevant, but the Bosch DME system as found in my Porsche requires a good number from the both the coolant temp sensor and the 'air temp' sensor (squirreled away inside the AFM meter) to operate an engine properly. Without it the injector firing pulse will be way off, and cars so equipped may not even start. My Jeep's CTS read 1000 Ohms with the engine 'cold', but with ambient temps in the high 90s.



Tomorrow...or later today..I'll take a closer look at the injector wire harness, and maybe try installing a different crankshaft position sensor with BOTH holes drilled out to see if I can bump up the output a little more. With one hole drilled out, and pressed inward when tightened, I get just .31V. It's a brand new Mopar part, and had previously put out just .25V. The vehicle does have the factory bypass harness on it, installed twenty years ago by me.



Any thoughts on what I've done so far?


Thank you for all your help!
 
I was only able to spend a little time with it today, but I had my first 'ah-ha' moment. I checked all of the fuel injector leads today, which all tested intact between the ECU connector and the connectors at the injectors. Highest resistance observed was .4 Ohms. It was when I checked the sensor ground circuit I found something amiss. I have opens on both the intake air temperature and coolant temperature circuits. The MAP sensor and TPS sensor ground circuits are intact. Oddly, I checked the resistance from the coolant temp sensor yesterday and got a good reading of 1000 Ohms. I took this reading at the ECU connector (back-probed). Now how did that happen if the ground side of it is open somewhere along the way??? Perhaps I was just seeing the resistance of the internal ECU board on this circuit....Tomorrow I should have more time and will verify signal legs of each circuit, and try and locate where in the harness my sensor ground leads have let go. I looked for a minute tonight and found a wad of duct tape around some wires, I think the fuel injector ground leads, where they all come together. I will get that straightened out regardless.



Will the loss of coolant temp and intake air temp signals cause a no-start on Renix???
 
I was only able to spend a little time with it today, but I had my first 'ah-ha' moment. I checked all of the fuel injector leads today, which all tested intact between the ECU connector and the connectors at the injectors. Highest resistance observed was .4 Ohms. It was when I checked the sensor ground circuit I found something amiss. I have opens on both the intake air temperature and coolant temperature circuits. The MAP sensor and TPS sensor ground circuits are intact. Oddly, I checked the resistance from the coolant temp sensor yesterday and got a good reading of 1000 Ohms. I took this reading at the ECU connector (back-probed). Now how did that happen if the ground side of it is open somewhere along the way??? Perhaps I was just seeing the resistance of the internal ECU board on this circuit....Tomorrow I should have more time and will verify signal legs of each circuit, and try and locate where in the harness my sensor ground leads have let go. I looked for a minute tonight and found a wad of duct tape around some wires, I think the fuel injector ground leads, where they all come together. I will get that straightened out regardless.



Will the loss of coolant temp and intake air temp signals cause a no-start on Renix???

Checked the website yet?
 
Checked the website yet?


Yes, I have. There's a lot of useful tips there, but I didn't see too much in the way info on symptoms of specific faults such as which sensors causing which faults, such as missing coolant and intake air temperature signals. Other reading I've done tells me the missing data should cause a rich condition, but not a no-start. I'm going to re-check the two sensor grounds, and loop check other circuits as well. I also want to figure out how to check the distributor pickup. I've found oddly contradicting information about it's importance. Some say Renix systems will run fine with it unplugged while others have reported no-starts.
 
You can unplug the sync sensor and forget about it.
I was service manager and shop foreman at a Jeep dealer from 1980 through 1992.....sync sensor never caused any issues. We only ever replaced them if the scanner showed they were dead and the vehicle was under warranty.
Biggest cause of crank-no start was CPS and/or C101 connector.
 
I thought that might be the case based on much of what I've read. I may look into it, just as a matter of getting it to run as smooth as possible.



I have to report I was DEAD WRONG about the open circuits on the temp sender grounds. I didn't realize that BOTH wires on the senders were brown. When I wiped the first wire and saw it was brown I went with it. I didn't even note the tracer because who in the heck would put a brown wire next to a brown wire with a tracer??? Well, JEEP, that's who! So..I was testing the wrong legs. They're both good.



One thing I had noted was the starter relay. The connections looked kinda grimy. I didn't think it was really an issue because I was getting good voltage from the junction there, and the starter cranked reliably, but I knew I should clean the contacts just the same. And if I was going to do that I wanted a new part in it's place. You know what? After I finished installing it, the damn thing started up and ran fine! I'm not sure I can declare victory just yet because I still haven't found what the problem was. My Jeep started fine a few weeks ago, then it didn't. And it has both had difficulty starting AND stalling out on me in the past. That it's running now only makes it difficult to figure out what's been going on. I was really hoping it would not start until I found the issue. So, right now I'm wondering...us there anything about the starter relay that can cause a crank-but-doesn't-start condition?


Thanks again for everyone's help!
 
Back
Top