Struggling with something that maybe someone here has been thru before.

Symptoms:
recurring Code 33/34/36 on a stock 89 coupe.

The first thing I did was the quickest and simplest, replaced both relays. Reset the ECM and all is fine...for awhile.

After about 6 or 7 start/stop engine cycles, the code reappears. Reset the ECM without doing anything and all is fine...for awhile. Then yup, after about 6/7 start/stop... you get the picture.

Now the engine runs absolutely fine. No problems starting, idling or accelerating. I have double checked the MAF wiring, jiggled the connector, made sure of a solid ground...don't know a thing about the ECM, but I'm really not thinking it's bad.

I haven't pulled the MAF sensor cable off while the engine is running to see how the engine reacts, but I'm mostly doubtful it's the MAF throwing the codes, UNLESS...

...at some point in all of this, I did clean the MAF and noticed it's not your standard MAF (at least like I've seen in all the catalog pictures). It has/had no front screen (didn't appear to have been made with a front screen) and instead of an internal tube with the wire inside, mine has a thin copper (?) plate dead center of the thing.

Again the car runs absolutely fine, but eventually keeps throwing the same codes...so I guess my question is this - is it possible that the MAF is different enough to eventually tick off the ECM while still working just fine?

The only other thing I can think of is a possible vacuum leak somewhere, but I doubt that too.

I don't have a problem resetting the ECM once a week, but I'd rather not have to.

Any help in resolving this goofy problem would be greatly appreciated.

Thanks in advance!!!

 

Listed below are the parameters used by the ecm for setting the MAF error codes. They are based upon plausibility monitoring of the MAF signal under various throttle position and rpm conditions to determine if the reported airflow is plausible based upon the other signals.

If your seeing all 3 codes being set, the common denominator is the MAF signal itself. Maybe just a bad connection. Check the wiring harness and connector at the MAF and ecm first.

Scan data would show what is happening with the signal and would be usefull to see if there is an intermittent signal problem.

From your description, it sounds like you have a Micro-Tech MAF. From my experience, these MAFs tend to over report the airflow under heavy throttle, but this tendency should not by itself cause the codes to be set.

I've also seen reports of the wire insulation shrinking back and exposing wire in the vicinity of the relay connectors. You may want to also inspect the relay connectors for this condition.


;--------------------------------------------------
; ERR # 33 Params
; >> MAF Sensor Hi <<
;--------------------------------------------------
LC20F FCB 38 ; If TPS >= 14.8%, set ERR #33
; CALIB = Arg * 2.56
;
LC210 FCB 45 ; If Air Flow <= 45 gms/sec, disable ERR #33
;
LC211 FCB 96 ; If > 0.6 Sec, Set code #33
; CALIB + Arg * 160
;
LC212 FCB 10 ; 1000 Msec ERR 34 Min time req.
; CALIB = Arg * 10
;
LC213 FCB 128 ; If RPM > Disable #33
; RPM/25
;
LC214 FCB 100 ; 10.0 VDC PUMP VDC FOR ERR 33 ENABLE

;--------------------------------------------------------
; ERR # 34 Params
; >> MAF Sensor Lo <<
;--------------------------------------------------------
LC215 FCB 0 ; 0049 COUNTS, IF ANALOG CNTS x 7 or
; PP2 DLT LT 0 SET ERR 34B
;
LC217 FCB 10 ; ERR #43, 1 Sec Min time req.
; cal + Arg * 10
;
LC218 FCB 24 ; If RPM > 600, Ck ERR #34B
;
LC219 FCB 16 ; If TPS > 6.25%, Ck ERR #34B
; cal = Arg * 2.56,
;
LC21A FCB 0 ; If LV8 >= 0, Ck ERR #34
LC21B FCB 130 ; If LV8 <= 130, Ck ERR #34B
;
LC21C FDB 0049 ; 0049 COUNTS, IF ANALOG CNTS x 7 or
; PP2 DLT LT 0 SET ERR 34B


;--------------------------------------------------------
; ERR # 33/34 Params
; >> MAF Sensor Hi/Lo <<
;--------------------------------------------------------
LC21E FCB 167 ; If TPS > 65.2%, then use for MAF Default
; CALIB = Arg * 2.56
;
LC21F FCB 21 ; Gms/sec Scale factor
; gms/sec/IAC Count
; CALIB = Arg * 256
;
LC220 FDB 1024 ; 4 Gms/Sec default air flow offset
; CALIB = Arg * 256

;--------------------------------------------------
; Default air flow offset per pct TPS vs RPM
; Value = Gms/Sec * 100 pct, (Offset)
;
; CAL = Arg * 100
;--------------------------------------------------
LC222: FCB 8 ; 9 LINE TBL
;
; Gms/Sec RPM
;-------------------------------------------------
LC223 FCB 30 ; 0.30 400
LC224 FCB 90 ; 0.90 800
LC225 FCB 110 ; 1.10 1200
LC226 FCB 135 ; 1.35 1600
LC227 FCB 150 ; 1.50 2000
LC228 FCB 175 ; 1.75 2400
LC229 FCB 210 ; 2.10 3200
LC22A FCB 225 ; 2.25 4000
LC22B FCB 230 ; 2.30 4800
;----------------------------------------------


;----------------------------------------------
; ERR # 36 Params
; >> MAF Burn off Diag <<
;----------------------------------------------
FCB 3 ; 0.3 Second Dly prior to Burn off
; CAL = Arg * 10
;
FCB 6 ; 6 fails req for ERR #36
; If HLM air meter
FCB 95 ; Fail B/O test if HLM A/D > 95
;
FCB 20 ; Fail B/O test if A/D < 20

 

Thanks for the reply and all the ECM parameter code threshold settings!! Outstanding information and as luck would have it, I had just ordered an ODB1 cable (http://www.aldlcable.com/) that I intend to use with the TTS Datamaster Diagnostic Software, so that is great info you passed. That should help answer ALOT of questions for me.

Also you nailed the MAF perfectly! It is a Micro-Tech! There was no label on my MAF (or any manufacture markings) but it matched up perfectly to their product on their website.

I do have another question for you tho...what use is the burnoff function playing for me right now? I can understand a thin wire heating up easily, but my MAF has significantly more copper and I just don't see it getting hot enough to amount to much burnoff. Would this be a problem?

Thanks again tequilaboy!!!

 

Good question.
Few years ago I installed a Wells MAF in the Vette; it had a PC board instead of the OEM thin-wire design, so I though no burn-off function possible.
Since I didn't like the higher gm/min MAF readings I swaped it with the EOM MAF in my '86 IROC-Z.

I prefer the OEM design MAF as all the original features are there and it's easy to diagnose.

You might pick up a used MAF from an F-body car in a bone yard and see if your code issues go away.

 

One of my kids had an F-body car with an erratic MAF. Also did not really perform as well as it should.
After many different shops and a really expensive trip to the dealer, their tech found a broken lead just at the entrance to the connector (on the car side of the connector). New connector and it ran like a scalded cat.
I don't think he would have found it without his scanner.