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 Help diagnosing intermittent EFI start problem 
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Post Help diagnosing intermittent EFI start problem
The last couple days I've been trying to figure out a start issue I've been having with my EFI. I'll try to describe the symptom as best as I can and what I've done so far to troubleshoot it.

Symptoms: Bronco will generally start no problem on first attempt first thing in the morning. After driving it anywhere from 1/4 mile or beyond and then having the engine off for say more than 10 minutes, the engine has a very difficult time starting IF it starts at all. This happened Saturday when I drove to a friend's house that lives in my neighborhood and then I couldn't start it up again to go home. Had to leave my Bronco overnight at the friend's house. When I returned yesterday morning to try and start it, it did start, but took more effort than it should have. Then yesterday I drove around the neighborhood stopped back home, turned the engine off and left it off for like two minutes, tried starting it and it started back up immediately. Howeverl I then tried to restart like 10 minutes later and it would not start and wouldn't start the rest of the evening. The kicker is, up until this weekend, this start --> drive ---> stop ---> no restart has been intermittent. I've been left stranded places on a few occasions and I'm over it.

So, here's where I am at so far in my troubleshooting:

Fuel pressure is where it should be

I'm getting spark

KOEO codes are 53, 67
CM codes are 53,96

And now I'm trying to figure why the 53 code is showing up. I tested the TPS a little bit ago and am confused by my results because I am getting 5.7v on the voltage source wire (orange) and 5.7v on the sensor output (green) wire. Shouldn't the voltage source wire read around 5.0v and the sensor output wire read around 0.5v and then increase as the throttle opens? Not sure if this is the cause of my start problems, but I do need to resolve the 53 code right?


Mon Jul 17, 2017 9:26 am
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Post Re: Help diagnosing intermittent EFI start problem
I just replaced my fuel pressure regulator. It's too early to tell if it's my issue, but it was very intermittent as well.

Fuel pump relay? Can you hear the pump(s) running with the key on? I assume so since you say the pressure is good. What pressure are you seeing?

TFI module? Where is it mounted?

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1973 Bronco, 351 SEFI, Locked, discs, 35's ZF-5spd and Atlas 4spd. 235:1 Crawl Ratio. It may be ugly, but it's slow.
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Mon Jul 17, 2017 10:40 am
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Post Re: Help diagnosing intermittent EFI start problem
Fuel pump relay is new and good--pump kicks on at turn of the key and fuel pressure reads 35 psi. TFI module is located on the distributor, which I purchased new from Jeff's Bronco Graveyard.


Mon Jul 17, 2017 11:06 am
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Post Re: Help diagnosing intermittent EFI start problem
Code 67 could indicate an open signal return circuit which would affect the TPS, BP, ACT and ECT sensors. What computer and harness are you using?


Tue Jul 18, 2017 10:10 am
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Post Re: Help diagnosing intermittent EFI start problem
Viperwolf1 wrote:
Code 67 could indicate an open signal return circuit which would affect the TPS, BP, ACT and ECT sensors. What computer and harness are you using?


Computer is an A9L and the harness is one made from a place called Chuck's Roadster Electrics out of Nevada City, CA.


Tue Jul 18, 2017 10:44 pm
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Post Re: Help diagnosing intermittent EFI start problem
cplynn wrote:
Viperwolf1 wrote:
Code 67 could indicate an open signal return circuit which would affect the TPS, BP, ACT and ECT sensors. What computer and harness are you using?


Computer is an A9L and the harness is one made from a place called Chuck's Roadster Electrics out of Nevada City, CA.


You should check the signal return line to see if it is actually going to ground. The ECM should provide that ground but you can ohm between one of the sensors and pin 46 of the ECM connector. If you get continuity there the problem is likely inside the ECM.

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Wed Jul 19, 2017 10:40 am
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Post Re: Help diagnosing intermittent EFI start problem
Good luck- I jut got a headache looking at that diagram!!


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Wed Jul 19, 2017 3:26 pm
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Post Re: Help diagnosing intermittent EFI start problem
Viperwolf1 wrote:
cplynn wrote:
Viperwolf1 wrote:
Code 67 could indicate an open signal return circuit which would affect the TPS, BP, ACT and ECT sensors. What computer and harness are you using?


Computer is an A9L and the harness is one made from a place called Chuck's Roadster Electrics out of Nevada City, CA.


You should check the signal return line to see if it is actually going to ground. The ECM should provide that ground but you can ohm between one of the sensors and pin 46 of the ECM connector. If you get continuity there the problem is likely inside the ECM.



I've been out of town for the last week so I just now got a chance to test this. There is no continuity between the sensor and pin 46 so that checks out. Any other ideas?


Sat Jul 29, 2017 3:46 pm
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Post Re: Help diagnosing intermittent EFI start problem
cplynn wrote:
Viperwolf1 wrote:
cplynn wrote:
[quote="Viperwolf1"]Code 67 could indicate an open signal return circuit which would affect the TPS, BP, ACT and ECT sensors. What computer and harness are you using?


Computer is an A9L and the harness is one made from a place called Chuck's Roadster Electrics out of Nevada City, CA.


You should check the signal return line to see if it is actually going to ground. The ECM should provide that ground but you can ohm between one of the sensors and pin 46 of the ECM connector. If you get continuity there the problem is likely inside the ECM.



I've been out of town for the last week so I just now got a chance to test this. There is no continuity between the sensor and pin 46 so that checks out. Any other ideas?[/quote]Well that's a problem. Are there any jumpers or connectors in the harness that could cause the non-continuity?


Sat Jul 29, 2017 4:16 pm
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Post Re: Help diagnosing intermittent EFI start problem
I guess I'm a little confused now. My understanding was that if continuity is shown to be present between a sensor and pin 46 of the ECM connector, there's likely a signal return grounding issue within the ECM. Based on this, my understanding was that the opposite--a non-continuity situation between a sensor and pin 46--would be OK. But you're saying since there is a non-continuity instance there's a problem somewhere else?

I checked for continuity between pin 46 on the ECM connector and the TPS and there was no continuity. Wouldn't this cause a no-start situation all of the time? My Bronco does start after a long rest period. It just won't start again within minutes of shutting the engine off. When I got back home from my trip I tried starting my Bronco and it started right up, no problem at all. I drove it around the neighborhood, parked in my driveway and turned it off. Tried restarting within 5 minutes and it absolutely would not start. The next morning, it started easily again.


Sat Jul 29, 2017 9:44 pm
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Post Re: Help diagnosing intermittent EFI start problem
Look at the diagram above. The sensor grounds are all tied to pin 46.

Without the sensor grounds you may be able to get it to start but it wouldn't run well. It's also possible that particuler harness ties those sensor grounds to a different ground point. It would be worth testing that.


Sun Jul 30, 2017 11:05 am
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Post Re: Help diagnosing intermittent EFI start problem
You could have more than one problem. Your symptom sounds like a pressure regulator with a leaky diaphragm to me. Pull the vacuum line off the regulator and see if there is any signs of fuel in it. The 96 code points towards a problem in the fuel pump circuit. Since the pump is new, look at the connector for the inertia switch, they are known for making bad contact and burning up. If all is well and the code shows again throw a new relay at it.

But first, "go with what you know". The missing ground / sig return to the TPS is an issue that you need to fix first. It could be reading WOT to the PCM while cranking and reducing injector pulsewidth enough that it won't start hot. Or, it may have nothing to do with the no-start, but you have to address it first.


Sun Jul 30, 2017 1:07 pm
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Post Re: Help diagnosing intermittent EFI start problem
Made some headway in resolving the codes, but still having the start issue. Turns out my TPS was bad. The spring-loaded piece of the TPS was no longer spring loaded for some reason. Luckily I had a spare TPS, which is testing properly on the multi-meter and I am no longer getting the 53 code.

I am now getting continuity, so that's good. I went back through my grounding points to assure good contact and I guess that resolved the continuity.

There's no fuel in the vacuum line of the fuel pressure regulator and the inertia switch connector looks OK.


Mon Jul 31, 2017 9:31 pm
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Post Re: Help diagnosing intermittent EFI start problem
An easy way to check the inertia is to wire in a jumper in the back side of the connector and put a switch in the loop. I did that only to find out the inertia switch doesn't appear to be my issue. They are so darn simple there isn't much to fail...

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http://www.ucora.org


Tue Aug 01, 2017 7:09 am
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Post Re: Help diagnosing intermittent EFI start problem
I am now pulling an 11 code, so I guess that's good, but still having the same issue. I'm about ready to use this thing for a boat anchor!

To make matters worse, I found out last Tuesday I lost my job and my last day of work is tomorrow, so I won't be able to spend any more $$ on my Bronco for a while, which means it may be out of commission longer than I had hoped.


Thu Aug 03, 2017 7:51 am
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Post Re: Help diagnosing intermittent EFI start problem
Sorry about your job! I hope you can find a new one quickly!

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http://www.ucora.org


Thu Aug 03, 2017 8:57 am
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