Codes 34 & 51 after fuel pump change, but no FPCU change

[Mikesinfiniti]

I mentioned in another post that I changed the fuel pump and the car fired right up and was warming up in the 1200 rpm range. It then dropped to around 700 or so and the CEL came on. I checked the ECU and it shows codes 34 and 51. I also checked the MAF and it is fine. Could this be FCPU related? I thought that since it started after the pump was changed, that everything was ok with it.

Please give me your best guess. I have limited time and tools and want to start searching for a FCPU if that's a logical path.

[goody90q45]

Any other symptoms other than the CEL being on? Missing, sluggish, black smoke, fuel smell, etc.?

[Mikesinfiniti]

No black smoke and starts fine. When it idles down to 700 rpm it runs rough when in drive. At cruising speed it seems to run ok.; no backfiring, fuel smell, etc. About 2 years ago I had a O2 sensor go bad and it ran the same way. Could it be the O2 sensor and coincidentally go bad after the pump change or could it be the FCPU?

I'm tempted to go pick up a FCPU and give it a try. Any thoughts?

[goody90q45]

A couple of weeks ago the owner of the 90Q I sold called with a code 51. It was loose female pins on the MAF connection. Used an eyeglass screwdriver to crimp down the female connections, cleaned and lubed both sets of pins, and the CEL went away. It's worth a look and will only take a few minutes.

Another member just had a code 51 and it was corrosion in the ECU connection. The point is it may not have anything to do with injectors but is probably another electrical issue. Good luck.

[Mikesinfiniti]

Mike,

Thanks. I did see those posts on the code 51 and mine MAF connection and ECU connention looks good. I'll check again.

I guess I'll roll the dice on the FPCU.

Anybody else with any ideas?

[Mikesinfiniti]

I just talked with my brother-in-law who helped with the pump change by buttoning up things when I was at work and he confirmed that he did not use the new o-ring (I found the new one in the trunk this afternoon - he told me the new one was installed, hmmmm; gotta love family!).

Could this be the problem if he used the old one - maybe a pressure leak at that spot which could cause the 34 and 51 codes?

Any ideas? Thanks.

Mike

[Q45tech]

Fuel pump or FPCU, as in low pressure, will not throw any codes. Why you must measure the fuel pressure with a gauge under ALL operating conditions.

The ecu assumes that the pressure will be correct [thanks to the FPR].

Codes in OBD 1 are simple affairs [some signal NOT no signal] [some voltage above zero but less than 5.0 volts] means the sensor is connected and thus is assummed to be accurate.

This assumed to be accurate is where drivability problems come in NO CODES just acting weird.

OBD2 adds back up sensors to double check each sensor and enough ecu smarts to know when the signal is incorrect [outside of +- 20% of what one could expect for the conditions.

Understanding the ZEN of each engine is important.

[Mikesinfiniti]

Thanks Q45Tech. I guess I'll check the O2 sensors and injectors while I'm waiting for the replacement FPCU.

Mike

[Q45tech]

Think about the fact that 43.4 psi is not much different than 34 psi:43.4/34= 1.2764 take square root ~= 13% more flow at same injector open time.

If the FPR didn't change [stayed at 34 psi] the engine would just run 13% LEANER.................instead of 10.5 AF ratio it might be 11.87 AF...........so if the FP or FPCU CANNOT provide enough VOLUME the pressure migh drop below 34 psi [even with a working FPR].............

Why measuring the PSI is the first step in any diagnosis. It should be at least an annual event to check the PSI.

In fact an electronic PSI gauge should be installed on every Q as another critical inside gauge!

[Mikesinfiniti]

Thanks for the math and engineering lesson. I never felt so inadequate. I guess what you are telling me is that I should be checking the PSI of the new pump to determine whether it's the FPR or the FPCU.

I don't know where to begin. So, I will change the FPCU when it comes and see if that works. If not, maybe I'll try the FPR and if that doesn't, off to the shop to fix it and then the car is up for sale. Don't really need the aggravation. We have 6 cars and 1 cycle in the driveway and can do without the Q, even though I have owned it for the past 9 years and would love to keep it.

Thanks again.