jthwan22

My car is having problem starting. It will crank hard but the fuel pressure reads about 15psi. If I pull the fuel pump relay and jump terminal 30 & 87 I can hear the fuel pump running and fuel pressure goes up to 60 psi. I can start the car with the jumper wire or take off wire and put the fuel pump relay back on.

The LH computer was rebuilt 6 months ago.

No fuel smell on the vacuum line off the fuel dampers or fuel regulator.

The leak down on fuel pressure as follow after shut down
0 min - 60psi
5 min - 52psi
10min - 48 psi
15min - 45psi
20min - 45psi
25min - 43
30min - 42

What should I check next?

Mike Frye

Did you say it will start with the relay jumpered, but if you put the relay back in, it won't start?

If you start it with the jumper in, does the pressure stay up where it belongs?

If so, I think you just found that the relay is the problem, no?

jthwan22

It will start with the jumper wire and relay (after the jumper wire build up fuel pressure)

If I just use the fuel pump relay It will crank but no fuel pressure.

Mike Frye

Did you try swapping the horn relay and FP relay to see if it builds up pressure (and runs) with that relay in place of the FP relay? I think they're the same one in your MY as well.

Seems like a weird failure mode, but swapping relays is one way to see if that's the problem.

jthwan22

Yes I got bunch of #53 relay from SITM. I think the relay is OK but not getting the signal to energize from the computer while cranking. Once it is running, it is fine.

So what is the part that tells the fuel pump to turn on? Crank positioning sensor?

Buler? Buler?

FredR

The fuel pump is initiated from the secondary circuit board on the LH unit. This is the bit that is prone to failure on LH units and is the main component that John Speake has improved when he refurbishes these units.

If this circuit board fails then a dodge is to externally trigger the fuel pump.

Regards

Fred R

jthwan22

The LH was rebuild recently. I wonder if it is still under warrenty? The jumper wire to #30 & #87 at the fuel pump relay position will get old real soon.

Landseer

Condition of grounds at back of valley?

(Both where the wires are soldered into the ring terminals and the terminals themselves to the valley)

Computers ground there. If control voltage is intermittent to fuel pump relay, then the cause may be inconsistent grounding.

Also if there is cross short / melt on back of CE panel, control voltage can be compromised. Saw this on an 85 panel once. I think the melt came from a defroster-related circuit wire that somebody had over fused. Hard to see, but it melted into wires it crossed.

You can start the car with the fuel pump jumper, then measure the other two terminals for control voltage profile.


Another thing that will do this is a wire melt inside the harness back by those grounds. If you manipulate the harness with long needle nose pliers and try to start it, it might disrupt the short and re-establish control voltage.

jthwan22

Why would the groud cause this problem? I will try to clean it anyway to see if it helps

Landseer

LH has two separate ground wires that lead to the six-wire ground MP VIII at the base of the bell housing. LH provides the trigger for the fuel pump relay. Beyond that, I don't know enough about the mechanisms to give anything more than antedotal evidence.

See also this thread and clip. As car would get hot, it would stall because fuel pump relay trigger would drop out, a short in the harness causing it. This happened both before and after MAF and LH were tested and rebuilt. MAF had a high temp circuit failure also, but after rebuild the car would still stall, but on a different frequency pattern!!! Layered problems.

So, lots can happen in the area of that short harness run on the 32V cars.

https://rennlist.com/forums/928-foru...ideo-clip.html

John Speake

LH has to see rpm pulses in pin 1, then it will energise the fuel pump relay. I doubt the problem is with your LH ECU...

It sounds as though there is a very large voltage drop to the fuel pump with the relay in place and engine cranking. Time to check all the grounds already mentioned. Also check the connections at the 14 way connector at the jump start terminal, and the smaller wires that are on the battery positive terminal. Also check the braided earth strap from battery to body.

Does the engine crank briskly or slowly ?

jthwan22

The engine crank briskly just won't fire up. Once it is running, it runs fine.

John Speake

OK, understood. That confirms good battery volts to starter, but not to CE panel when cranking.

As Landseer suggested , check voltages to the coil side of the FP relay. There should be igntion switched 12v on one side of the relay coil, connect that to ground via a load (i.e. sidelight bulb 5W). Swicth on ignition and check there is a good 12v there.

The other side of the FP coil is grouinded when the LH ECU receives the rpm pulse on pin 1, but rather harder to test that. If you can insert a test wire with the relay, you could see if there is any voltage difference on that pin compared to a chassis ground with a voltmeter, when engine cranking.

NOTE, be very careful with connection to the FP relay socket, if you connect a jumper to the wrong pins you can easily burn out the FP driver stage in the LH ECU.

With LH ECU removed check resistance to chassis ground from pins 5 and pin 17 of its 35 way harness plug.