Another crank no start thread but with a twist

[Jhalsey]

New to this forum and hoping for some help. I have a Gen II Viper engine with a starting problem but its not in a Viper chassis. It’s a Viper crate motor and PCM in a 1972 Datsun 240Z chassis. Long project but its been reliable of late but the other day it stranded me. Parked had breakfast and when I returned it fired right up as usual and then stalled. I could tell right away something was amiss, it never stalls. It fired again and I tried to drive away but again it stalled. Attempted restart, this time it cranked but didn’t even kick. Waited an hour, tried again, no luck so flat bedded home.

At first I thought it was a fuel problem the way it “gradually” died but I have a permanently installed fuel pressure gauge in the engine compartment. With key on I can hear the fuel pump run and the gauge shows 55 psi. I checked the schrader valve in the fuel rail and good spray when depressed. So I’ve dismissed the fuel supply as the problem.

I have the Mopar Viper Crate Motor manual and it explains the start sequence w PCM looking for a crank signal and a cam signal as preconditions to starting. I had replaced the cam position sensor a couple of years ago so I thought I would inspect the crank position sensor which is the original now some 25 years old. Pulled the sensor and w key on, I see 5.2 v on the outside pins and zero on the center connector. I believe the center connector is ground but oddly the center connector shows 48.2 Ohms between it and chassis ground. Not sure what to make of that, perhaps there are some dirty grounds or some harness damage somewhere. I don’t have a scope so no means of testing the sensor. Ordered a new cps, cheaper than a scope.

The wiring on this car is difficult to access, I spent an entire winter on my back head up under the dash and have no wish to repeat that. Pulling the dash is also a bear. With the dash out, I installed two relay centers w fuses on the inside firewall , a total of ten relays w switched and fused outputs. The ASD relay is one of these as is the fuel pump relay. In retrospect, I should have done something more easily accessible. In any case, if the new cps doesn’t cure the problem, looks like I’m destined to pull the dash.

The car has 10k miles on and was built in California. It has a Tremec 6 speed (Tranzilla) and Datsun IRS w Quaife LS. Firewall is moved back 4” and has a full cage. It drives beautifully, when running, torque everywhere. Suspension is coilovers and Wilwood brakes all around w electric brake power assist and electric steering assist.

I will try to attach a couple of photos for your viewing pleasure. If anyone has any ideas for trouble shooting the no start condition, I’m all ears.

Thanks

 

[Goggles Pizano]

Disconnect plug and retest plug. Back probe it too connected.

 

[Jhalsey]

Double checked the readings on the open connector. Same as before. Also back probed the connector. Same readings.

I am now attempting to partially disassemble the the engine bay wiring harness to see there is a short or other damage. This is the factory crate motor harness and there is a large connector on top of the bell housing. If I can get that open, I should be able to find the CPS pins and test for continuity.

 

[Vic]

Do you have any fault codes?

 

[Viper Specialty]

Contact me. This is the kind of thing I live for. LOL

Off the cuff guess, you have a Standard PCM installed, not a Crate Engine PCM... and it has come around to bite you in the ass. That, or you have a CKP/CMP issue.

 

[Jhalsey]

Fault codes would be nice but though it has a diagnostic connector, PCM does not support fault codes, or so says the manual.

Dan, thanks for offering to help, I will call you. It DOES have a crate engine pcm and wiring harness. Only stranded me one other time and that that was a fuel pump fuse. Car now has 10k miles of road use, no track sessions. Not familiar with CKP/CMP, what is it?

 

[Goggles Pizano]

In reference to the sensors
CKP = Crank position
CMP = Cam position

 

[Goggles Pizano]

You can check the ground pin on the MAP or throttle position sensor as they are easily accessible as they all tie to the same pin on the PCM as the ckp and cmp. The ground signal for all of them is on the same circuit. It is NOT chassis ground but PCM ground. There is a big difference hence why you are getting 48 ohms

If you do continuity between sensors it will show close to 0 ohms

 

[efnfast]

If you hold the gas pedal to the floor when cranking will it start? (i.e., add more air)

2yrs ago I re-tuned my 01. Worked perfectly fine. Then last year I went to start it and it would not start. Only crank crank crank, no fire. When I held the tbs opened then it would all of a sudden start just fine. So we adjusted my startup tune for more air and ever since then the car's started and driven perfectly fine.

Why all of a sudden it wanted more air, and only at startup, i don't know. You'd obviously think mechanical, like a leaking injector, but i went through everything several times and found nothing mechanically wrong/changed

 

[Jhalsey]

Ah, crank and cam positions, thanks for clarifying. With regard to will it start with the gas pedal to the floor, I haven’t tried it. At the moment, I have several sensors disconnected in my efforts to trace the 3 wires from the CPS back to the PCM while waiting for the new CPS to come from China. I will give it a try when the electrics are all buttoned up, although my tune has not been changed in many years. The main mystery is why is there 5V on the CPS sensor pin when it is disconnected from the sensor?

 

[Goggles Pizano]

...The main mystery is why is there 5V on the CPS sensor pin when it is disconnected from the sensor?

pull down design. Read up on it.

Plus did you check what I posted?

Don't bother with the gas pedal to the floor. Vehicles are engineered to have a 'flood clear' mode that cuts fuel on cranking when activated by WOT on crank.

 

[efnfast]

pull down design. Read up on it.

Plus did you check what I posted?

Don't bother with the gas pedal to the floor. Vehicles are engineered to have a 'flood clear' mode that cuts fuel on cranking when activated by WOT on crank.

gen 2 vipers don't have a clear flood mode (I'm guessing the crate engine ecu doesn't either). Or don't hold it to the floor, just stab the gas a bit to open the throttle body while cranking to see if it starts when adding air.

basically just try starting it by adding more air to see if anything changes; it's no more difficult than watching the fuel pressure gauge when cranking

 

[Jhalsey]

so I checked and have continuity between the ground pins of the CPS, the TPS and ATS (air temp). I also hear the fuel pump run for a moment after I release the start button and it stops cranking.

 

[99RT10GTS]

These engines/cars are known to have issues if the cam Position Sensor goes out, no codes thrown, turns into a guessing game. I am having a similar issue on my 2001 RT, first thing I do when I get back to it is swap the cam sensor. If you find the issue, would love to know what it was.


Pop that hood and take pics. Also, if you decide to sell, message me

 

[Jhalsey]

I will post pictures of the engine bay. I replaced the cam position sensor maybe two years ago. The water pump had failed and in the course of replacing it I mistakenly removed the CPS. Of course when I subsequently put it back I installed it incorrectly and when I started started the engine, it was destroyed. Had to pull the pan to clean out all the debris and change the oil and filter. Major fiasco, but the result was a new CPS. Hopefully it is still good.