spark, fuel, compression, tach bounce but no start
#1
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spark, fuel, compression, tach bounce but no start
Hello, and thanks for reading. As a 944 owner I know that usually we would be off bungee jumping or with Scarlet Johanson in a bubble bath but you took time out of your day anyway. Again Thanks
First things first:
- Yes I have used the search function, for most of the day in fact
- I have spark
- good fuel pressure
- normal compression
- The tach bounces slightly as I attempt to start the car
- the belts look fine
- DME relay looks fine, operated correctly and was inspected by my electrical-engineer-neighbor an hour ago
- cranks no problem
- no lights on the dash
- good battery voltage
- Air meter is free moving
- no fuel escaping from the vacuum port of fuel PSI regulator
Last night, the car ran great, drove home like a champ including a few pulls over 6500 rpm. No loss of power, nothing out of the ordinary.
I jumped in the car this morning and it cranked sounded like it wanted to start but didn't. I established the above and now am hoping that someone has the words of wisdom that will allow me to go to work in the morning
This should be a decently satisfying thread as I am a veteran mechanic and have all the tools to test theories. Please help if you can
Thanks, Greg
First things first:
- Yes I have used the search function, for most of the day in fact
- I have spark
- good fuel pressure
- normal compression
- The tach bounces slightly as I attempt to start the car
- the belts look fine
- DME relay looks fine, operated correctly and was inspected by my electrical-engineer-neighbor an hour ago
- cranks no problem
- no lights on the dash
- good battery voltage
- Air meter is free moving
- no fuel escaping from the vacuum port of fuel PSI regulator
Last night, the car ran great, drove home like a champ including a few pulls over 6500 rpm. No loss of power, nothing out of the ordinary.
I jumped in the car this morning and it cranked sounded like it wanted to start but didn't. I established the above and now am hoping that someone has the words of wisdom that will allow me to go to work in the morning
This should be a decently satisfying thread as I am a veteran mechanic and have all the tools to test theories. Please help if you can
Thanks, Greg
#2
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If you have spark and fuel at the rail, that means the injectors are either not firing, or not getting their ground pulse from the DME. I'm currently troubleshooting the same thing. Its either the injectors themselves, the injector harness, or the injector driver in the DME. or some combination of those 3.
#3
Drifting
Put a noid light (cheap at auto parts store) on input lead of injectors and see if you get good pulses. Check to see if the rotor is held in it's proper position on the distributor shaft. The screw can back off causing mis-timed ignition. Also check to verify the dist cap is held firmly on with the two twist screws. Good fuel pressure is one thing, sufficient fuel flow is another. This can be affected by dirty fuel tank strainer, collapsed fuel return line or contaminated fuel. That's what I have so far.
#4
Rainman
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i gave scarlett the day off, so here i am.
im with doug, the problem might be with the injectors/wiring, but since none of them are going, its more likely the wiring than an individual injector.
pulling the fuel rail is a 5 minute job and you can pulse each injector with a drill battery and see if gas comes out.
im with doug, the problem might be with the injectors/wiring, but since none of them are going, its more likely the wiring than an individual injector.
pulling the fuel rail is a 5 minute job and you can pulse each injector with a drill battery and see if gas comes out.
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Alright The update for today:
- noid light was not triggered on any injector
- continuity test revealed that all injectors (both poles) have 0 resistance to BOTH pins 14 and 15 (both banks of injectors)
- they also have continuity to pin 35 (idle positioner)
- the harness appears to be in tact
- with the key on all injectors have power
Questions before I hack open my harness:
- Does the fact that both poles of the all injectors have continuity to both injector drivers mean there is a short?
a) if so have I destroyed the drivers in all likelihood?
b) Has anyone performed this test in the past with differing results?
- Should the injectors have continuity to pin 35?
As always and Ideas are greatly appreciated
Thanks, Greg
- noid light was not triggered on any injector
- continuity test revealed that all injectors (both poles) have 0 resistance to BOTH pins 14 and 15 (both banks of injectors)
- they also have continuity to pin 35 (idle positioner)
- the harness appears to be in tact
- with the key on all injectors have power
Questions before I hack open my harness:
- Does the fact that both poles of the all injectors have continuity to both injector drivers mean there is a short?
a) if so have I destroyed the drivers in all likelihood?
b) Has anyone performed this test in the past with differing results?
- Should the injectors have continuity to pin 35?
As always and Ideas are greatly appreciated
Thanks, Greg
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#8
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Going through the same thing, you need to recheck the harness with the dme and all injectors unplugged. If any of the injector plugs have continuity between the two pins, the harness is bad. If not, the dme is bad.
#9
I assume this is an 8 valve engine. With the key on there should be 12 v at each injector. The injectors are fired by the DME giving the other pole of the injector a ground. Injector power does not come from the DME. I reread the post. I see you have power at the injectors. Looks like pins 14,15 ground injectors. Check continuity through harness. 15, injector 1 and 2. 14, injector 3 and 4. Looks like bad harness or DME. Probably DME.
#10
Rennlist Member
It always helps to mention what year and model car you are dealing with
Noid light not firing on the injectors is a pretty good clue of where to point your investigation to. As you've probably figured out, the injectors fire when pins 14 and 15 on the DME ground themselves. If you've got 12 volts on the injectors with the key on and known good continuity from the injectors to pins 14 and 15, then maybe the DME is unhappy. But these DMEs are pretty robust and usually don't just fail from one day to the next. If you could swap in a known good DME you could rule that possability out.
Might be time to go through and clean all the grounds.
Noid light not firing on the injectors is a pretty good clue of where to point your investigation to. As you've probably figured out, the injectors fire when pins 14 and 15 on the DME ground themselves. If you've got 12 volts on the injectors with the key on and known good continuity from the injectors to pins 14 and 15, then maybe the DME is unhappy. But these DMEs are pretty robust and usually don't just fail from one day to the next. If you could swap in a known good DME you could rule that possability out.
Might be time to go through and clean all the grounds.
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Thank you one and all for the input.
The car is an 87 944 8v NA, with around 360,000 miles
harveyf:
I would love to swap in good DME but the nearest one on car-part looks to be about 250 miles from my house so i'd like to keep that for a last option.
joes and dougs951s:
My concern is that not only does Injector 1 and 2 have continuity to pin 15 but also pin 14. Similarly Injectors 3 and 4 connect to 14 as the should, but also connect to 15. Would you say this consitutes a harness failure?
The car is an 87 944 8v NA, with around 360,000 miles
harveyf:
I would love to swap in good DME but the nearest one on car-part looks to be about 250 miles from my house so i'd like to keep that for a last option.
joes and dougs951s:
My concern is that not only does Injector 1 and 2 have continuity to pin 15 but also pin 14. Similarly Injectors 3 and 4 connect to 14 as the should, but also connect to 15. Would you say this consitutes a harness failure?
#12
Rennlist Member
It's possible (probable?) that you have a broken solder joint in the DME. Definitely not uncommon on these cars and the problem can manifest itself without warning. Would be worth your effort to pull the DME and have a look.
#13
This is the way I suggest that you do this. Find the other end of 15 at #1 or # 2 injector and ground it. Make sure you get the right one at the injector (Key off) Unplug the DME find pin 15 in that plug check for continuity through 15 and the chassis. Then take the ground away and the continuity should go away. Do the same for the rest. According to the drawings no way should 14 and 15 be together. Now if the DME is plugged in when you were checking I don't know what kind of results you would get. Send your ADDY and I'll send you the drawings.
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Update:
-I was testing the harness to pins 32 and 31 aka ground pins So the harness works and I'm a moron.
- Testing between injectors 1 and 2 goes right to 15 and 34 for right to 14 all have .03 ohms of resistance
- I pulled the DME apart and have found all kinds of brownish yellow stuff peeling off the board (water corrosion?!)
Questions:
- how do I locate the injector drivers
- Is .03 ohms of resistance reasonable over the harness? or could that inhibit injector function (I ruled this out because this was a sudden event that stopped the car and the deterioration of a wire is a slow process. Also it could be accounted for by a lose connection of the ohm meter but I am happy to be corrected )
Thanks again one and all. resolution is so close I can feel it.
-I was testing the harness to pins 32 and 31 aka ground pins So the harness works and I'm a moron.
- Testing between injectors 1 and 2 goes right to 15 and 34 for right to 14 all have .03 ohms of resistance
- I pulled the DME apart and have found all kinds of brownish yellow stuff peeling off the board (water corrosion?!)
Questions:
- how do I locate the injector drivers
- Is .03 ohms of resistance reasonable over the harness? or could that inhibit injector function (I ruled this out because this was a sudden event that stopped the car and the deterioration of a wire is a slow process. Also it could be accounted for by a lose connection of the ohm meter but I am happy to be corrected )
Thanks again one and all. resolution is so close I can feel it.
Last edited by drive135mph; 08-26-2014 at 07:54 PM.
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Oh and before it is suggested;Yes I did see the tech article about DME's I am not sure how to determine whether or not my two injector drivers are damaged, don't get me wrong I'm scouring the board looking for any clues
Last edited by drive135mph; 08-26-2014 at 08:02 PM.