348 RPM sensor question

Hello my fellow 348 brothers. I have some questions for you. My '91 348 TS has thrown some codes. Here are the codes and here is what I have done so far.

Codes:

Bank 5-8
1121 RPM sensor
1211 O2 sensor or coolant sensor or MAF
1212 O2 or MAF

Bank 1-4
4121 Cat temp ECU
1121 RPM sensor


I have already:

1) Replaced both left and right O2 sensors.
2) Added a ground wire to the left O2 sensor.
3) Had the right MAF rebuilt (by DirectAuto)
4) Swapped the Ex ECUs (no affect)
5) Installed new spark plugs (NGK)
6) Inspected both RPM sensor connectors to make sure the pins are making proper contact (all good).
7) reset the main ECUs (disconnect battery and do the 10 minute start up).


The car:

1) bank 1-4 spits and sputters at idle.
2) misses when driving at constant speed
3) backfires out the pipes and the airbox after letting off the throttle. (appears to be from bank 1-4).
4) runs strong under full throttle.
5) both cats are running under 500 ºF (checked with infrared temp gun)

Questions:

1) Does the same RPM sensor trigger the CEL for both banks or does each sensor feed each bank?
2) If both banks are giving the RPM code, does that mean I need to replace both of them or just one?
3) If just one RPM sensor needs to be replaced, which one is it? The one on the bottom or the side?
4) Since bank 5-8 is giving two codes that include the MAF and I have already replaced the O2 sensor, does that suggest the left MAF? I can swap the right one to see if that moves the codes to the right bank.
5) Does it sound like the thermocouple on the left cat is bad?
6) Do you recommend that I spray some carb cleaner inside the left MAF?


Thanks so much for taking the time to go through all of this stuff. I did not pick up the RPM sensor code until I happened to check codes without the engine running. I did not realize until after further reading that you could only get this code with the engine off. Thanks in advance for the feedback. Todd

 

Thanks No Doubt for the response! So you are saying that you could possibly get the RPM sensor code if you check them with the engine off even though it is good? I think that I misunderstood what you stated on your blog. I thought that you had to check the code with the engine off in order to check to see if the RPM sensor was bad. After re-reading it again, I now see what you meant. Ok, I will report back after working through your suggestions. Thanks again! Todd





Did I mention how awesome this forum is?

 

Ok, I cleaned the MAFs with MAF cleaner. I installed a ground wire on the right side O2 sensor. I reset the ECUs. It now runs great! No sputtering. The only code that is still there is the 4121 on bank 1-4. No lights come on at all (CEL or slow down). I did swap the exhaust ECUs before but I may have been watching a different problem. Now that this is the only code shown, I am going to swap the ex ECUs again to see if the 4121 switches sides. I guess that if it does not switch sides then the problem is with the thermocouple sensor right? Does it hurt to drive it this way with this code if the cats are not getting hot? Thanks No Doubt for the previous feedback.

 

Thanks for the words of support! I swapped the Exhaust ECUs and the code also swapped over so there you have it. Still no CEL or slow down lights and the cat temps are just fine. When I decide to get this ECU rebuilt, is BBA-Reman the best choice at the moment? Is there anyone in the US that also rebuilds them? Thanks again for all your help! I do have to say that I enjoyed every moment working on my car. As long as this type of stuff does not happen very often, I do not mind it at all. I am looking forward to my next oil change.

 

Well, BBA-Reman told me that they do not rebuild the exhaust ECU's. The only thing they could offer was to try and find a used one for me. Does anyone know of a company that is rebuilding the exhaust ECU's? Thanks

 

Hello Nick, thanks for the reply. You may have not read far enough down in this thread. I at first thought that I was having an RPM sensor problem because I was getting the code for it. But No Doubt let me know that you can get this code by mistake if you check codes with the engine off. I already checked the wiring and connectors as well with no issues. I checked the codes again with the car running and there were no RPM codes. What actually fixed my sputtering problem was connecting a ground wire to the O2 sensor. I was also getting an MAF code as well on the other bank so I (with No Doubts suggestion) cleaned it with MAF cleaner spray. Now the car runs great. The only issue I have now is an error code from the exhaust ECU. The problem is with the ex ECU and not the car. I swapped the ex ECU's and the code switched sides as well. I do not get any CEL or slow down lights so I will just drive it until I find a place to rebuild this bad ex ECU. I had seen a suggestion to have BBA-Reman rebuild the ex ECU but they told me that they do not do that. They only rebuild the main ECU's. I was hoping that someone had a suggestion for another company that does rebuild the ex ECU's. Thanks

 

My ex ECU is sealed (potted). Both of them are this way. There is no cover to remove. Or am I missing something somewhere? This looks like it was done at the factory. It is not just filled with silicone. Is this a sign that it has been rebuilt already?

 

Ok, I understand. Mine are sealed in black so I guess they are the older version. So what should I try? Should I take the car out for a drive to dry up any moisture in the ex ECU and then cover the black sealing material with a coating of silicone? Or is it too late? Thanks for the help.

 

I will take a look at it when I decide to replace it. Do you think it is too late to keep it from setting this code? If I seal it up, do you think it will go away? I am going to try and seal it first. I will take some pics of the unit before I put any silicone on it. Thanks for the help guys.

 

Ok, I took the troublesome unit off to inspect it. At first glance, I thought that there was nothing wrong. But I looked closer and found a small separation along one side of the casing and sealing material. You can see this area circled in yellow in the pic. I then took a heat gun and blew on it for a couple of minutes to try and dry any moisture out of it. After that, I put a coat of the blue sensor safe silicone around the edges. I let it dry and reinstalled it. It has been raining all afternoon so I was not able to test it. I had just spent 4 hours washing and waxing her so I was not about to go right out and get her dirty. I will let you know if the code is still there as soon as I can test her out. Thanks
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I hope it heals itself now that I have sealed it. It has been raining every day since I finished it so I still don't know if it will set the code again.


~Boomer Sooner!!!

 

Well, the good news is that the 4121 code is gone from the 5-8 bank after sealing up the ex ECU on that side. Now I am getting a 4122 code from bank 1-4. I pulled the ex ECU from that side and it was in worse condition than the other one. I could cleary see the cracks along the sides of the sealed area. So I again, hit it with a heat gun and applied a coat of silicone sealant. I will test it tomorrow and see if I am finally code free.

 

Ok, now after sealing up the ex ECU on bank 1-4, the 4122 code is gone but now I am getting the 1211 code also from bank 1-4. It is also idling rough now. It does not sputter or backfire like before. The idle just fluctuates up and down periodically. I have already replaced the O2 sensors on both sides so I am going to remove the MAF and clean it with spray to see if it gets better and the code goes away. More later.