Fuel Pump, FPCU and check engine codes 34 & 51, etc.

[Mikesinfiniti]

Well, I picked up a FPCU form a bard member and got the same check engine codes - 34 and 51. Decided to check the injectors (code 51) and all 8 injectors measure 20.8-21 ohms. I figured that since they were all about the same I'd check the knock sensor and the left bottom pin measured 578 ohms and the right bottom pin mesaured 1900 ohms. Not sure what that means but after reconnecting the all of the connectors the check engine light was off and then I took it for a ride. Seem to run well. Got back in the driveway with the engine idling and revved it a bit and let it idle for a few minutes. The rpm's dropped to about 500 rpm and began to run rough. The check engine light came back on and the same code appeared.

Any ideas? Could it be the knock sensor?

Thanks for any help.

Mike

[goody90q45]

Knock sensors don't light the CEL so the code 51 is the culprit. The KS reading in the 550 range is good, the other in the 1900 range is going south on you. I've been driving with a dead KS for over a year and my Q idles fine and accelerates great. If you're ever under the plenum for any reason (see next paragraph) you should replace both KS and so should I.

As far as all your injectors reading in the 20.8 range I would suspect the accuracy of your voltmeter (DVM). You can verify your DVM accuracy by checking against a ten ohm resistor purchased from Radio Shack. It should read very close to 10.0 ohms across the leads of your test resistor. If you checked on a warm engine try it again when things cool off. The resistance will go down. You can also check direcly on the connection of injectors 1,2 and 8. If they really are reading that high it will be time soon to replace 8 injectors.

Did you tighten up the female pins on the MAF connection?

[Mikesinfiniti]

I'll check the MAF to make sure, but the FSM says that code 34 is a knock sensor problem. I replaced one of the O2 sensors about 2 years ago and it was running the same way, but I don't recall if the CEL was on. Could it be the O2 sensor? I guess I need to dig through the FSM and see how to check it. By the way, I checked the injectors at the connector as described in Q45.org. I have another tester, so I'll check them again.

I assume the key should be on and everything else should be connected, correct?

Thanks for the help.

[goody90q45]

You don't want the key on to check the injectors or KS. Once you find out what's causing the code 51 and fix it the CEL won't light up again. You'll continue to get the KS code, which doesn't light up the CEL, until you replace them. The code will just be stored in the ECU.

You've really got to check the tightness of the MAF female connector pins. I had code 51 (running rough, black smoke) a few weeks ago on my 90Q and that's all it was. When I bought the 90Q I had code 51 (leaky injectors) and it was bad O2 sensors. In both these cases the ECU code doesn't tell you what the real problem is. Too bad. Good luck with your gremlins.

Modified by goody94q45 at 6:02 PM 12/17/2006
Modified by goody94q45 at 12:54 PM 12/21/2006

[Mikesinfiniti]

Ok, I finally got a chance to tear the MAF assy apart and cleaned it as per the instructions in Q45.org and greased the connection. Also, did the injector and knock sensor harnesses. Took the car for a ride and have the same CEL and the same codes. I calibrated my ammeter and got the same results on the injectors - all in the 21 range. And the KS's are still showing 578 and 1900. I guess I'm down to the Right knock sensor as the culprit, but I know it's a stab in the dark.

I seem to remember reading about changing the oil filter and removing it through the top of the engine. Since I've owned the car at 30,000 miles, I've always done this. Could someone tell me what the harm is in doing this. Could I have damaged a connector in the area. I normally unscrew the filter, plug the hole with my forefinger, and pull the filter up through the fan blades, rotating the blades around until the filter comes out.

Thanks for any assistance. More than likely if I can't get the CEL off, I'll be selling the car for what I can get - $1000, $500, etc just to be done with the headache. If anyone is interested, e-mail me at [email protected]. I am located in eastern Pa, Bucks County.

Mike

[elwesso]

Mike when you checked the injectors did you do it with everything in the car off. Seems weird they would all give a high ohm reading....

Right now im not concerned with the injector reading so much as Im concerned about the CEL.. HOWEVER the fact that we're getting a really high resistance reading could mean something.

what I want you to do is check the resistance on the harness... im wondering if maybe your injector harness is bad... That would be a bummer but its not a real hard part to replace neither is it terribly expensive! I would take off the harness connector to the number 1 or 2 injector (under that plastic thing in the front center of the motor) and ohm test the injectors directly and see if you get a good reading... IF you get a good reading at the injector, you can then test the harness... if you have the FSM turn to that section and test the continuity and resistance of that section of the harness..

[Mikesinfiniti]

Thanks Wes, I'll give it a go this weekend if I get time. I checked the injectors at the connector with the engine off, but the key on. I just drove it and it seems to run fine at speed (w/the CEL on), but runs a little rough at idle.

The car was running great before the pump failed. My gut tells me it is an O2 sensor - possibly the one of two that I did not have changed some time ago. It's acting the same way as it did when the other O2 sensor was bad (Or the knock sensor because of the high reading from the right side?). I guess I'm getting a little tired of the trial and error approach.

Thanks for the help. I'll post results.

Mike

[elwesso]

Ahh the key being on is why your injectors were off... Turn the car COMPLETELY OFF and then retest....

Thing is code 51 is for INJECTOR CIRCUIT and code 45 is for INJECTOR LEAK, 45 normally means bad o2 sensors but the ECU is actually seeing theres something wrong with the injector circuit...

[Q45tech]

An outside of bounds rich signal can be a real rich signal MAF or injector leak OR a false rich signal from slow switching or frozen O2 signal

[elwesso]

But those wouldnt throw a 51, right? That indicates something wrong with the injector CIRCUIT, not the ECU assuming something is wrong because its rich on bank vs the other.

[Mikesinfiniti]

I wish someone could explain to us layman. I'm sure that there is more than just me that "doesn't get it"?

I'm still thinking a bag O2 or knock sensor. Could that be it and is there any way to know without trial or error?

Thanks again for the help.

Mike

[elwesso]

We need to address why the CEL is coming on FIRST... I think thats obvious to everyone...

Code 51 is for INJECTOR CIRCUIT.. There is a seperate code for what you could call injector function, which is code 45... Code 45 is often misleading, but thats not what we're talking about.

Code 51 comes on when "the injector circuit is open or short circuited"... So, that means that the ECU is seeing an abnormally high (probably 12v) or abnormally low (0V) voltage reading from one of the injectors....

So what do we do about this? We check the harness continuity between each injector.. The FSM has us testing other things first but after looking at them, im not sure the car would run if those prior things werent as they sould be... We can skip to checking continuity on the ECU harness, as its easy enough to check for continuity on the injector harness itself... However, you are getting ohm readings on each one so that seems OK... RECHECK your OHM readings with the ignition switch OFF and report back! just to verify the integrity of injector harness itself...

If you have troubles, FEEL FREE to call me sometime (tomorrow)... My contact information is on the contact page of Q45.org

[Mikesinfiniti]

Ok, I checked the ohm readings on the injectors and knock sensor with the key off and here is what I found:

LEFT prong of KS: 578RIGHT prong of KS: no reading

INJECTORS:

#1 -6 & 8: 11.5-11.6#7: no reading.

So, what do you think? By the way, as I mentioned I did check with the key off, but the other day I inadveratantly left the courtesy lights on inside the car and the battery went dead. So the above readings were with the key off and the battery charger hooked to the battery and charging.

Thank you for your help.

Mike

[elwesso]

Well obviously that means your #7 injector is completely dead! Fortunately you can get at the #7 injector without taking too much apart.... I just went out and looked at it and at a minimum you can get the connector off by removing or moving the IAC out of the way.... Id just take it off...

Once you take the IAC off, take both probes of your VOM and put it on the pins on the injector after you disconnect the connector... See what it reads.. if you get some sort of reading at that injector (regardless of if its in spec or not) then you know your harness is bad on that circuit, if you get the same thing then you know your injector is bad... you can test the harness continuity on this circuit too, but try what I said first and we'll move on from there.

[Mikesinfiniti]

Hi Wes,

I finally got a chance to pull the connector off on the #7 injector and I get no reading at the injector pins with my VOM. I assume that is my problem? Also, it looks like an injector change on #7 requires a lot of hardware removal - intake manifiold, hoses, etc. Is there a shortcut w/o having to remove the intake?

Please advise. Thanks.

Also, does anyone have any injectors for sale for a 94 Q45? If so, please e-mail me at [email protected]. Thx.

Mike
Modified by Mikesinfiniti at 4:05 PM 12/30/2006

[maxnix]

Injector 7 (last one on dirver's side bank) should come out without too much problem and not require removal of the plenum.

But you really don't want a used injector unless you want to repeat the labor again very soon.

http://www.infinitipartsusa.com

[Q45tech]

Code 51 comes on when "the injector circuit is open or short circuited"... So, that means that the ECU is seeing an abnormally high (probably 12v) or abnormally low (0V) voltage reading from one of the injectors....

All injectors have +12 volts applied every time the key is onThe negative terminal of injector goes to ecu where a switching transistor grounds it to complete the circuit. [just like with coils except the a closed and open when the coil is to fire.

The ecu has NO way to check this switching transistor nor if the injector has 12 ohms nor if it is drawing the normal 1 ampere current. A lower than 10 ohms injector will overheat transistor and cause it to fail....too much current.

Everyone of the injector codes is generated by GUESSING there might be a problem thru software routines...............O2 is the only signal that can be used to see if a cylinder fires. other than a drop in rpm when it should be steady.

OBD1 is dumb as rocks..........it does what it is supposed to but does know the results............

Reading codes is just the start [the very beginning of the process] of the diagnosis......................you must then do series of other measurement test to zero in on the real problem.

OBD2 has 2 sensors for every function this an OBD2 code may be 98% accurate vs a OBD1 code which is 50/50 if that.

[maxnix]

I seem to remember reading about changing the oil filter and removing it through the top of the engine. Since I've owned the car at 30,000 miles, I've always done this. Could someone tell me what the harm is in doing this. Could I have damaged a connector in the area. I normally unscrew the filter, plug the hole with my forefinger, and pull the filter up through the fan blades, rotating the blades around until the filter comes out.

The problem is the MAF connector usually gets worn out or abused, especially the pre late 1994 (final configuration) ones.

Best always done from underneath so you can inspect the undercarraige and suspension for wear and leaks.