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Gary, ASE Certified Technician
Category: VW
Satisfied Customers: 1180
Experience:  Senior Technician
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I am experiencing a no start condition where there IS fuel

Customer Question

I am experiencing a no start condition where there IS fuel and spark.
Submitted: 2 years ago.
Category: VW
Expert:  Juan Crespo replied 2 years ago.
Hi! I’m Juan and my goal is to help you solve your car issue.
Have you tried starting it by spraying a little choke cleaner or starting fluid down the open throttle? If it starts and runs for a few seconds, then what's probably happening is that the ECM is not opening the injectors to let fuel in the engine. Can you try that and let me know? Also, could you tell me what kind of diagnostic tools/equipment do you have access to?
Customer: replied 2 years ago.
Yes I did, and it did not start, I bought a new fuel pump and installed it as well. I changed all the relays in the relay box under the dash. The fuel pump does turn on as I can hear it. The rear seat is taken off.
Customer: replied 2 years ago.
I have an OBD2 scanner that I purchased but it says there is no error codes, what is really weird is there is absolutely no sort of engine light. From my understanding an engine light should be present at least during prime cycle.
Customer: replied 2 years ago.
I checked for codes again and I am getting a ton of codes. Here is the list as follows: p0753, p0758, p0763, p0768, p0773, p0778, p0748, p0725, p1857, p1850. Hopefully this helps.
Expert:  Juan Crespo replied 2 years ago.
Thank you for that information.
The check engine light (MIL) is controlled by the engine computer (PCM) and should come on every time the ignition is turned on - no MIL means there is a problem with either the PCM or its circuits. The used PCM you got from the yard is either bad or not a perfect match.
On your first post you indicated there was spark and fuel; I'd just like to remind you that to get fuel into the combustion chambers the fuel pump has to deliver fuel to the injector rail AND the PCM has to open the injectors.
All of the P07XX codes are related to the transmission. The two P185X codes indicate there is no communication between the PCM and the transmission computer (TCM). Since the PCM and TCM are matched to each other, any replacements would also have to match (same part numbers as the old ones), otherwise they'll have to be programmed.
Kindly accept and rate my answer now if you think the information I've provided has helped you solve your issue. Otherwise, please reply.
Best Regards.
Customer: replied 2 years ago.
So even though both PCMs have the same codes and serial number they can be different? I was under the impression that the flashing of PCMs were done based off the "features" of vehicles and engine model. I have the original as well. So once a P185x code is set it is safe to say that the PCM is the culprit? No other wires or electrical switches can cause this fault in the communication of the PCM to TCM?
Expert:  Juan Crespo replied 2 years ago.
Let's clarify a few things before we proceed:The first issue that needs to be addressed is the no-start. The engine won't start if the PCM is not opening the injectors, regardless of how good fuel pressure and spark are. To check if this is actually happening, unplug any injector and plug in a Noid Light in its connector. Crank the engine; if the Noid Light does not flash brightly, the PCM is not actuating the injectors.The PCM and TCM need to be matched in programming with each other for them to communicate and work correctly together. Right now, codes P1850 and P1857 are telling us those two modules are not communicating.Buying parts from a junkyard is a gamble unless you actually get to see the part working when tested on another like vehicle. Kindly accept and rate my answer now if you think the information I've provided has helped you solve your issue. Otherwise, please reply. Best Regards.
Customer: replied 2 years ago.
Let's take the junkyard PCM out of the equation. Is the original one damaged based off of your diagnosis? I just want to know how to rule the PCM faulty. Could it be possible that a P185x code can set based off of something else?I have tested the fuel injector connector with a meter. I don't have a noid light but on the meter it reads 4.7V with key switch ON. When cranking the meter shows 3.8V and it jumps in number range of 3.8V to 3.9V, back and forth.
Expert:  Juan Crespo replied 2 years ago.
I'm sorry if I'm not being clear enough.
To answer your first question; the reason I said the PCM or its circuits were bad was based on the fact that you indicated there was no MIL with ignition ON. I'm also saying that you need a Noid Light to test injector pulse to verify whether or not the PCM is or isn't activating the injectors - the ground signal from the PCM to the injector is very fast and short, a regular voltmeter cannot capture it. Advance Auto sells the Noid Light you need for less than $14.
Now, the answer to your second question is no; each data trouble code (DTC) number describes a particular issue with a particular circuit - in this case the issue being "TCM cannot communicate with PCM".
I am a Certified Master Technician with 30+ years experience and I will do my best to assist you, but you have to keep in mind that, while I can provide you with technical guidance, working on this type of issue may require you to have not only access to several different automotive tools, but the skills to use them as well.
Customer: replied 2 years ago.
The noid light does light up. I went ahead and replaced the crank sensor as well. I rescanned the computer and the P185x codes are gone... only codes left are P07xx codes. Any ideas?
Expert:  Juan Crespo replied 2 years ago.
OK, now I'm confused. Why would you replace the cranksensor if the noid light is flashing? Also, am I to understand the engine is now running?P0725 means the TCM is not receiving engine speed signal; P0748, 753, 758, 763, 768, 773 and 778 all mean there is an issue with the solenoids in the valve body. All of these codes combined mean there is lack of communication between the PCM and the TCM.Based on the information you've provided so far, my advice at this point is to find a donor car similar to the one you're working on (identical engine, transmission, accessories and features) and replace both the PCM and TCM as a pair. Kindly accept and rate my answer now if you think the information I've provided has helped you solve your issue. Otherwise, please reply. Best Regards.
Customer: replied 2 years ago.
I had read on a VW website that changing the crank sensor may solve the P185x codes so I changed it. The car is not running still. Are there any pins that I can check at the TCM and PCM that I can verify? And I will pick up a TCM and PCM pair and get back to you.
Expert:  Juan Crespo replied 2 years ago.
That's the downside of going by things people post on forums; most times the information is inaccurate. As far as I know, there are no pins you could check without a lab scope or high end multimeter capable of readings MIN MAX voltages.I believe I've done on my end all I could do to help you. At this point, I'm going to opt out and re-open your question so other experts may have the opportunity to answer. Please do not reply or the site will revert the question back to me. Best Regards.
Expert:  Gary replied 2 years ago.
Hi there,
In most cases, an ECM or PCM swap will require a relink to the immobilizer.
Have you done that?