adamtarr

Hi friends... I have attached a file for you to look at if you're interested. Would love some help with this as I'm already out thousands trying to fix this beast.

I'm troubleshooting a 1990 motorhome with a 454 tbi and a 1228747 ECM. Consistently experiencing what I think is a lean backfire though the TBI under load around 1750 rpm. Also occurs in idle at 1750 or so when I quickly depress accellerator. Finally got logging with WinALDL and have some good info to look at. However - I'm having a hard time deciphering what's going on in the file.

What is RAW:MW2 and all the "RAW" columns in the spreadsheet? Like RAW:Knock Count readings of 3?

Any tips are appreciated...

Thanks!

RFmaster

Adamtarr

I took a look at your posted idle data log. It appears that your ECM is not going into closed loop. This is indicated by both BLM and INT locked at 128 counts. '8747 ECM was used in HD applications (I guess motor home can be considered HD chassis). Some of HD applications do not use knock sensor KS and SA tables tend to be very conservative to keep engine out of spark knock. I would start by checking fuel pressure on that beast. It is not uncommon for in tank FP to get tired after years of service. Another (very common) possibility is that in tank fuel line is leaking (between FP outlet and feed through). What bothers me after looking at your log is that INT and BLM are locked at 128, yet both O2 oscillation and cross counts are there. Can you post your BCC (from 1228747) label?

//RF

adamtarr

Thanks so much for the reply. I did check fuel pressure and it's (or was when I checked) at 16psi. It will rev under load in open loop without backfiring and injectors are spraying. I can remove the wire from the CTS and the injectors dump fuel like crazy and it idles high.

Other scan tool says it's going into closed loop after CTS gets warm. This motor does have a knock senor.

Odd thing about fuel gauge - it does not get higher that 1/4 tank no matter how high the fuel level is. I removed fuel pump, operated float by hand and same issue. Checked power = good. Ground = good. Replaced pressure regulator and cleaned injectors.

What is INT and BLM?

I can post BCC if I know what that is...?

Thanks so much - I'm trying anything I can find.

RFmaster

Most GM Fuel gauges are designed to work with fuel level probes that utilize 90 Ohm potentiometer (aka variable resistor). Typically 0 is used for empty and 90 Ohms for full. You may have defective potentiometer or 30 Ohm unit. You can easily verify this DVM and moving float manually between two fuel level extremes.


On the language:

BLM = Block Learn Mode (aka Long Term Fuel Trim) - BLOCK LEARN term can be viewed as a semi-permanent automatic adjustment of the ECM Volumetric Efficiency tables. BLM values provide adjustment over stored value as long as battery is connected to the ECM. If battery is disconnected ECM has to re- learn fuel trim over what was stored in EPROM.

INT = Integrator (aka short term trim) is a short-term fuel adjustment variable used by ECM to compensate for rich or lean condition as detected by O2 sensor. Integrator increase or decrease the longer detected AFR is above or below the 14.7. Since ECM constantly varies AFR about 14.7 when operating in close loop the integrator provides immediate lean - rich indication to ECM.

I know that this a very short explanation - suggest that you read couple of stickies in the technical section.

BCC = broadcast code. It is a 4 letter alpha code used by GM to designate calibration used in your ECM. It should be on the same label with ECM part number - for example AMUR, ASDZ, etc

The reason one scan tool is detecting close loop is that one of the fault flags is set to 1, but BLM and INT are locked at 128. You may want to consider getting a more commonly available 1227747 (truck) ECM - it has been extensively reversed engineered.

//RF

SB406

Is this a direct plug in, pin for pin?

I have a 7747 harness and ECM in my truck and it had a mechanical speedo with an optical VSS on the back of the speedo head.

My buddy's 8747 harness has an eletronic speedo (no cable). I can't find a 8747 wiring diagram anywhere...

Thanks,
mike

RFmaster

7747 and 8747 are pin compatible. There are some differences in calibrations and some 8747 applications did not had KS/ESC input (harness). IIRC in 89 moon-eye cluster had a built in VSS that feed into a primitive DRAC (to provide pulse signals to CC and ABS). TBI ECM's operate of 2000 ppm square pulse VSS signal. How this signal is generated, from ECM perspective, is irrelevant.

//RF

SB406

Awesome. Thanks. Exactly what I needed to know.