'90 Renix Not Injecting Fuel - A Bit Long...

[OneOverZero]

I’ve just finished reassembling my MJ after a solid year of partial restoration and am ready to get it inspected.  When cranking, all I get is

– no fuel is being injected.  After probably a minute total of cranking over several hours, the cylinders and spark plugs have no traces of fuel.  It provided a healthy pop when starting fluid was sprayed.

First thing I did was pull the feed hose (feed and return are -6AN Aeroquip) from the ’94 rail and verify that it was providing sufficient supply when priming at key-on.  All seems good. 

The CPS (new, so untested) appears to be functional as the spark plugs fire consistently.

Injectors are reman/matched Bosch units, though I’ve also tried the old Bendix units since they’re handy.  When checked, all of both sets provided impedances that should be accepted by the ECU.  Although my multimeter can’t get a reading from the short pulse width provided by the ECU, it does display quick nonzero voltages when probing the injector pigtails while cranking.

Fuel pump seems to be functional – previous owner installed it in September 2011 (~1500mi) and all I did was pull it to replace O-ring and lock ring when I installed the new rollover valves/grommets and new injection hose & clamps.  It primes the system nicely at key-on.

All underhood electrical grounds are new and I cleaned/regreased all just in case.  I also cleaned out the bulkhead connector.  All electrical systems work apart from the interior lights (may need to finish some underdash wiring) and the radio (which illuminates but doesn’t turn on – probably also a minor issue).  All exterior lights, chimes/buzzers, and non-mechanical gauges function correctly.  All sensors are new (though I wish I'd left the old ones in for the time being).

This leaves me completely puzzled, but with a few possibilities.

1. Bad injector ground(s).  I can’t imagine that all of the signal wires have failed, though (if I remember correctly) the 1990 electrical manual refers to each injector grounding independently to the ECU.  The 1987 diagrams show a common ground – I thought these were identical systems, but I guess the transition away from the C101 connector changed things.
2. Zero/insufficient fuel pressure, possibly due to failed FPR (also new/untested), though I thought it would simply fail at the 31psi under-vacuum position.  When bleeding the Schrader valve on the rail, it always has an air pocket that reappears very quickly and the valve never produces a strong spray, so I'm suspicious that the FPR is just looping fuel back through to the tank.  But wouldn't the injectors do something, even under low rail pressure?
3. Fried ECU.  I originally had the battery ground going to the block, though this changed when we noticed that the ’87 Renix troubleshooting manual clearly shows the battery grounded to the starter.  Doing this resulted in a very hot ground and a clicking starter (why it’s different from the manual and why the starter isn’t grounding through the block is beyond me).  Returning the ground to the block fixed the issue and it cranks smoothly, but would the ECU be susceptible to damage with the bad ground?

My next step is to put a gauge on the fuel rail and confirm that it’s pressurized properly.  If that isn’t the case, I’ll swap out the Renix ECU for the Mopar-flashed TSB ECU, but is there anything else I’m missing?  Any well-known Renix issues that could be behind this?

Thanks!

Bob

 

a pre-disassembly shot...

[87manche1]

Do u actually here the fp run? Pressure test first. Check fp relay

[1987Comanche]

  You can hear the pump prime but can't hear the pump running while cranking (environment is too loud).  As soon as the key is released, however, you can hear the pump run for 0.5 sec or so then shut off. 

 

 A few questions/comments:

 

1.  Has anyone had firsthand experience in frying a Renix ECU?  If so how did you do it and what should be double/triple checked before plugging in the Mopar reflashed ECU?  These are pretty scarce and it would be very difficult to replace.

 

2. Does spark at the coil/plugs conclusively verify the CPS is functioning properly?  I thought the ECU had to see 300-400 RPM before the injectors would fire but I don't remember seeing anything about conditions for spark.  

 

3.  Assuming one could find an appropriate DRB II connector would a scan tool be of any help above & beyond a multimeter?

 

4.  Has anyone used a noid light set on a Renix injector harness?  If so, how did you fare out?

 

 5.  The continuous formation of air pockets and lack of spray from the Schrader valve is troublesome.  Can someone check their Renix and see if depressing the Schrader valve (Key on, engine off) produces a dribble of fuel or a spray?

[cruiser54]

Test the CPS:

 

Renix CPS Testing and Adjusting

　

　

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8" from the stock 5/16", or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

　

　

Revised 01-26-2013

 

Fuel should be SPRAYING out. Get a fuel pressure tester and check the pressures.

[87manche1]

Mine runs at 37 psi and bleeds down to 20 something. I would get spray from the scharder. May also look into the rubber line in the tank corroding away. New fuel filter?

[zackcj7]

 Do you have fuel in the rail after attempting to start? A friend of mine had a similar problem. He crossed his feed and return lines from the tank to the fuel rail. The feed should be the one on the back of the rail and the return on the front.

[comanchedude]

 Do you have fuel in the rail after attempting to start? A friend of mine had a similar problem. He crossed his feed and return lines from the tank to the fuel rail. The feed should be the one on the back of the rail and the return on the front.

YUUUP.. good point..

[OneOverZero]

Thanks folks!  NAPA should have my pressure gauge tomorrow, but I'll be out of town this weekend so this'll have to wait until Tuesday.  Definitely have some things to discuss with 1987Comanche over lunch tomorrow...

Mine runs at 37 psi and bleeds down to 20 something. I would get spray from the scharder. May also look into the rubber line in the tank corroding away. New fuel filter?

 

Yup - the only old components in my fuel system are the tank (cleaned inside, painted outside), '94 rail, and the ~2yr-old fuel pump.  Hoses, fittings, filter, seals, etc are all new.

 

 Do you have fuel in the rail after attempting to start? A friend of mine had a similar problem. He crossed his feed and return lines from the tank to the fuel rail. The feed should be the one on the back of the rail and the return on the front.

I've more than once second-guessed myself on this.  :yes:  The return is in-line with the FPR and the feed is connected to the second fitting (just behind the return on the '94/95 rail).  I checked this again tonight just in case, since confusing an identical pair of braided hoses isn't hard... fuel supply surges forth from the feed nicely.  Here's what they looked like last month before I buttoned up all my lines, hoses, wiring and such:

 

 

Test the CPS:

 

Renix CPS Testing and Adjusting

　

　

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8" from the stock 5/16", or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

　

　

Revised 01-26-2013

 

Fuel should be SPRAYING out. Get a fuel pressure tester and check the pressures.

This is something I should've been doing right off the bat before even attempting to fire it - should've done it when I checked the other sensors.

 

I'll have new information Tuesday.  Thanks again!

[1987Comanche]

Has anyone ever played with the "engine start" input to the ECU?  OneOverZero and I found a reference to this in the FSM.  Pin C3 & C4 on the ECU if I remember correctly

[cruiser54]

Has anyone ever played with the "engine start" input to the ECU?  OneOverZero and I found a reference to this in the FSM.  Pin C3 & C4 on the ECU if I remember correctly

I don't see a C3 and C4 in my manual. Please verify and tell us what you think this will accomplish.

[1987Comanche]

Verified for C3 and C4...

 

 

  So here's what I hope to accomplish.  If the ECU doesn't see the engine start signal, how does it know to execute the engine crank procedure? That would seem to be a key input...once again wondering if anyone has run into trouble here.  Lots of stuff listed on bad CPS, wasn't able to find anything on this.

[cruiser54]

Where do c3 and c4 get their inputs?

[1987Comanche]

Voltage provided to the starter solenoid during cranking is split.  One leg goes to the solenoid, the other provides the ECU input at C3.  Haven't figured out where the splice is yet though...

 

  Not sure if C4 is a constant ground or only provided during cranking.  OneOverZero and I ran out of time before figuring this one out. 

 

  What manual do you have?  The 1987 Electrical (orange with fold out pages) and 1990 Electrical (paper copy or the one Biotex posted) are the ones we've been using.