Red light comes on for 3 seconds then off when key is turned on

 

A P0191 is usually thrown when a specified pressure requested by the ECM is not observed at the FRP sensor. This is usually caused by a loss of pressure in the fuel system, lazy fuel pump, clog in the fuel system, or bad FRP sensor.

So, what i am gathering with what we know at this point is that the ECM is not inhibiting the fuel pump, but rather is transmitting a signal and expecting a pressure of 55 psi at the fuel rail. For some reason the pump is not receiving the signal, but is, for some reason, receiving the prime signal.

 

Do you have the diagnostic procedure for the code? Or a wiring diagram of the sensor to the ecm?

 

Mine does this normally as well (but I don't know if it is normal).

You will find the information for the DTCs in the "'How To' / DIY Repair & Maintenance" sticky, section six, at the beginning of this forum. You will find the electrical guides in section 7.

In page 26 of DTC powertrain summaries, it says that the P0191 is a 2-trip MIL, but when my check-valve failed this code was displayed about ten seconds after I cleared it, everytime. So, for me, it was not a 2-trip MIL.

 

What is a 2 trip mil? I have the service manual and diagram but there is something I'm not understanding about it.

 

MIL (malfunction indicator light). Two trip just means it has to record the same error on two consecutive drive cycles before it shows an error. The amber light will illuminate and log a DTC. For me, it would happen during the first drive cycle after clearing the code. This was caused by the failure of my check valve which released pump pressure into the fuel tank.

 

I have found once the code is set it will not allow the fuel pump to prime then soon as I clear the code the system will prime. I'm using a snap on scanner on the car.

 

Hmm, okay. I'll think about it some more. Let us know if you find something else that seems noteworthy.

 

The signal wire to the sensor has 4.2 volts plugged in to the sensor and 5 volts unplugged to the sensor. From my experience signal wires should have voltage not being plugged in. Can anyone give me some insite on this? Am I shorted somewhere causing the voltage?

 

Sorry to ask again, but did you measure the FRP using a scanner or a mechanical gauge? I suggest doing both. If the pressure value from both methods agree then your sensor is probably working properly. The pressure you are reading from the scanner is the value your ECM is receiving.

I need to know what you mean by 'signal wire.' I have not troubleshot the FRP sensor in these cars, but this is how one typical works. It is actually a voltage transducer. It takes a supply or reference voltage at the input, which is probably 5 volts. The fuel pressure applies force to an internal diaphragm causing it to deflect. The amount of deflection is converted to an electrical signal of varying voltage, in the case of the FRP transducer, probably somewhere between 0.5 and 5.0 volts. I would expect the actual upper range to be around 3.5 volts under normal operating conditions, but again, I have not measured this particular sensor before.

Something I should have mentioned before. If you haven't already, make sure all the grounds have been thoroughly cleaned and have a good connection. Also make sure your battery reads 12.6 volts with the car switched off. These are essential. Another issue that needs to be addressed if it hasn't already is to clear all the drains. This is especially true with your car, since it has already suffered catastrophic failure due to corrosion (maybe blockage from the cabin filter drain). There are numerous informative posts that address the drains and grounds. If all this has been completed, then please disregard.

These are all basic things to check. I am thinking the source of your problem is more complex, but it is critical that we make sure all of the fundamental factors are correct and accounted for, before we can take a deeper dive.

I hope all this helps.

 

Sounds like the fuel pump is working if it is priming. That probably means the fuel pump relay is also working fine. Just to make sure, I'd start by pulling the fuel pump relay (REAR POWER DISTRIBUTION FUSE BOX – R6) and jumping it to make sure you can get the pump to power continuously. (There should be a return line, so it should just pump up to the rail, past the fuel pressure regulator, and then back to the tank. Don't run it for too long though.) You might even try swapping it with another similar relay near by just as an added check.

Assuming that works, then I'd use a simple test light to check the signal wires to the fuel pump relay to see if you're getting a signal. Sounds like you'll see the signal to prime when you first turn the key on. But after that first 3 seconds or so, you need to see if you've got power through that relay to power the pump when trying to start. It sounds like you will not.

If you don't have a signal to the relay when you're trying to start, then you need to track down why you're not getting a signal or where it's getting blocked. I think the next component up stream is the Rear Electronic Module, but if you're getting a signal to prime, then I don't think that's the culprit. I'd probe the ECM next during starting to see if it's even sending a signal to power the pump. I believe that's pin 27 on the ECM connection, but you might double check that. I think you can test that with a simple test light, but you could use a meter if you prefer.

If you've got a signal coming from the ECM during starting, but you're NOT getting a signal at the relay, then there's a problem in between and maybe you do need to check that Rear Electronic Module. (On the Rear Electronic Module, I THINK pin cr11-19 is the signal, cr12-08 is for the priming, cr73-01 is for power on while operating, and cr73-02. -03, and/or 04 are the fuel pump grounds, maybe?) But if you don't have a signal coming from pin 27 of the ECM when you're trying to start, then it sounds like an ECM problem (or at least something is telling the ECM not to power the pump).

That's a start at least. If you can report back on that then maybe that will lead to something new. Good luck.

 

... found this, hope it helps.

 

No, interestingly it is a returnless system. The FRP transducer 'tells' the ECM what the FRP is and the ECM 'asks' the pump for more or less pressure.

The differing outputs for priming and running signal for the pump that you mentioned, Jeremy, is very helpful.
I am wondering if there is water ingress where the Rear Electronic Module is. I would make sure the connectors are clean and not corroded.

 

you obviously don't know these cars. There is no return fuel on them and the fuel pump relay is non serviceable

 

Good to know. I've had half a dozen X300's but I'm on my first X358. With that in mind then, I guess the next step is to probe the ECM to see if it's sending voltage to power the pump while starting. If it is, then look between the ECM and the pump starting maybe with the Rear Control Module circuits and grounds.

If it's not, then need to figure out why: bad ECM, corrosion, bad transducer, etc.? Looks like it should be easy to put a meter on the transducer to see how it's acting.

 

You should go back through and read all the post so you know what's been done.

 

Thanks so much, Erik, that's a great idea. Let's go through it:

1. He had the ECM reprogrammed..
2. The pump will prime when the ignition is switched on, but no fuel pressure on start.
3. Don't know why.
4. Replaced FPT on a hunch, but still no start.

So, what's missing? Why no pressure at start? Is it the pump? Not likely since it primes. Is the pump getting voltage at start? I think maybe you've checked but not 100% sure. But likely not because it appears to be working on prime.

To be sure if you haven't checked during start and while it's dying, it's worth checking because that's just a quick, easy test light check. But if it is not getting voltage on start (as expected) then why?

Is it the relay? Probably not because, again, it primes. So what else is there? The two prime suspects would be 1) broken circuit somewhere, or 2) a control issue.

A broken circuit could be a bad wire, bad component, bad ground, etc. anywhere in the system. You could trace every single thing, but it might be helpful to narrow down where the problem might be. Plus, if it's a control issue, that's going to make tracing the circuit more difficult.

A control issue is a little easier to trace or at least to rule out. As you suggested, reading the thread shows the ECM was recently worked on and the car still hasn't run since. That could be a red flag.

So, before tracing the entire system, is the ECM even sending the signal or "request" for pressure? Reading the thread it looks like we don't know. It's easy to test if you identify the right pin. (Is the pin even there? They've been known to bend, break, or pull out when being disconnected.) Looks like there's only one coming from the ECM (for fuel pressure). What would that tell us?

If the ECM is NOT sending the request, then it's not necessarily a bad ECM, but it would narrow down the cause and make it completely unnecessary to trace the circuit downstream. For instance, reading the thread, the FPT has recently been changed. It's probably good but if it's not or if the signal from the FPT to the ECM is not getting there, then that would cause a no-signal from ECM problem. So test for signal into the ECM. But, if the ECM IS sending the request, then it's probably pointless to check the FPT circuit and we should instead look downstream at something like the RCM or relay or grounds, etc.

So, after reading the thread again, I'm still at: 1) check to see if you're getting voltage at the pump on start AND while dying (I think you've done that but not 100% sure), and 2) check to see if the ECM is sending the signal on start

Was there something else in the thread that points another direction? Thanks again for the suggestion.

 

What I am gathering is that the ECM is transmitting a a request for fuel pressure, but the pump is not satisfying this request leading to a P0191 DTC.

Since the pump seems to function fine it seems that request signal is not reaching the pump. I've asked about this several times with no response, but we need to know if the FRP transducer is working correctly (I know it's new so it ought to be) by comparing its output to that of a mechanical gauge fitted to the schrader valve on the fuel rail. I have also mentioned the possibility that the fuel cutoff switch has been triggered or has failed, but I didn't get a response for that either.

Based on what Erik has told us and without any further knowledge, I am of the belief that water ingress took out the engine harness and ECM. I suspect further damage has occurred to other components since this kind of neglect is often widespread, i.e., multiple clogged drains, and each drain, when clogged, seems to have associated electronics that are at risk of water damage.

I have also asked about the usual suspects, ground and battery, but I haven't received a response, so I don't know the status of those either.

Anyway, without answers to some of my questions, I can't really contribute any more suggestions or guidance. Hopefully he finds the source of the problem.

 

any codes in the rem? looks like you might have a p1235 stored in the rem if rem wasn't receiving input from ecm . any other control modules have any codes in them?

 

Just unplug the FPR. The car will default to a high but usable fuel pressure. This will eliminate the FRP completely in your troubleshooting. Good luck with the replacement keys and cluster. This is a disaster as you are finding out.

Stay with the factory setup and properly program additional keys if needed.
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