williammackean

'85 4+3 w 68k miles.

Pulled a Code 34. It happenned as I went out in the 28 degree weather to go for a ride. Would not start without my foot slightly on the gas. Pump primes as per usual. It ran well, but only if I kept the RPM over 1000. Foot off the gas = stalled.

I cleaned the MAF connections. No help.
I have a MAF sensor test harness, but I don't know what the readings should be. Anyone know?

Performed a test:
Ran engine @ 2000rpm for 2 minutes.
Code 34.
Cleared.
Disconnected MAF.
Ran engine @ 2000rpm and immediately threw an SES light.
Engine ran roughly.
Code 33.
Bad MAF?

sami85L98

Here is what code 33 and 34:

CODE 33

Trouble Code 33 indicates that the Mass Air Flow (MAF) sensor is reporting more air is entering the engine than makes sense based on RPM and TPS. The MAF sensor produces a frequency output; around 30 Hz at idle to 150 Hz under acceleration. The frequency varies proportionally to airflow. The ECM monitors the frequency and thus determines airflow into the engine. Typical idle MAF is 5 to 7 grams per second.

The conditions for setting this code are:

no Code 21 or 22 (TPS Error) present, and
the air flow reported is > 40 grams per second, and
TPS indicates < 10% throttle position, and
the engine speed is 1400 RPM or less, and
the above conditions exist for over 5 seconds.



Typical causes for this code include:

1) Faulty MAF-to-ECM connection
2) Exceedingly noisy spark plug wires
3) Poor routing of MAF harness (i.e. near coil packs)
4) Maladjusted TPS sensor
5) Defective MAF sensor
6) Defective ECM


Return to Chart


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CODE 34

Trouble Code 34 indicates that the Mass Air Flow (MAF) sensor is reporting less air is entering the engine than makes sense based on RPM and TPS. The MAF sensor produces a frequency output; around 30 Hz at idle to 150 Hz under acceleration. The frequency varies proportionally to airflow. The ECM monitors the frequency and thus determines airflow into the engine. Typical idle MAF is 5 to 7 grams per second.

The conditions for setting this code are:

no Code 21 or 22 (TPS Error) present, and
the air flow reported is < 4 grams per second, and
TPS indicates 10% or more throttle position, and
the engine speed is 1800 RPM or higher, and
the above conditions exist for over 5 seconds.



Typical causes for this code include:

1) Faulty air ducting to or from MAF sensor
2) Faulty ECM-to-MAF connections
3) Poor routing of MAF harness (i.e. near coil packs)
4) Maladjusted TPS sensor
5) Defective MAF sensor
6) Defective ECM


I fought with 33 for a long time, it finally went away with replacement of burn off module, i dont know about ur case, also i do have occasionally code 51 (ECM bad),

U need to check these things,
burn off module,
tps voltage: set to .54 at idle,
IAC, : Clean both inside iac housing ans as well as IAC itself,
check MAF (disconnect harness and take a ride, if it makes any difference sure ur maf toasted),

At the same time clean TB too with TB cleaner (stp is a good stuff).

Hope someone will chime, RRT vette and 65Zo1 is pretty good guys who mastered in these and also others too.

williammackean

I just replaced the IAC and cleaned the TB & plenum. Been driving for a couple weeks and this started up.

It first materialized as a stalling after a 30 minute ride. It threw no codes then. Last night it was idling wild as I moved it ~10 feet.

Then it would occasionally idle a lot lower than usual. I normally idled at 690. Recently it dropped to 600-ish from time to time after a 30 minute run.

Today, the problems.

I wonder if I shouldn't get the Power and Burnoff relays on my way home tomorrow. Best to try the $20 fix before the $250, huh?

I wish I knew what voltages are correct at the MAF without the engine running.

I'll doublecheck the TPS setting once she idles.

Thoughts?

(Firewalled at work, so I'll be offline during business hours.)

RRT vette

From our buddy AGENT86

Just follow the directions, and you should be good.

http://members.shaw.ca/corvette86/Code%2034.pdf

williammackean

Did it. (I printed this and went to the Vette and followed the flow chart a couple hours ago.) Hence my thoughts of the MAF.

The catch is, tho. Point #1 states that the MAF disconnected should run OK, but the MAF connected should stall. I stalled both ways.

Would run @ high RPM ok with MAF connected. Would sputter at high RPM with MAF disconnected.

Am I over thinking?

sami85L98

Before spend heck of a money that MAF **** aint cheap, do the cheap stuff first, Clean MAF with Electronic cleaner and stp TB cleaner and Tps Re-set and IAC cleaning with same TB cleaner both inside housing and pintle,

AGENT 86

Unfortunately, the 85's don't have those relays, instead there is some sort of MAF module in the dash, and it ain't cheap. Finding a flow chart for an 85 would be a good start. PM Central Coaster, Im sure he has the info for your 85. Good luck

sami85L98

Only 85 came with burn off module, its very expensive and its a pita to replace, its behind the bread box and u need to open the dash and had to fight to take it out,

williammackean

TPIS says the 85 has the relays. When I mentioned that I was told about some module behind the dash, they said that's not true.

I don't know who to believe, so I'm headed for the engine bay to see where the wires go. What do the relays look like in an 86? These are on the driver's side of the disty on the firewall, right?

If I do in fact have the 'module,' where do I get one?

RRT vette

Didn't think of it before when I posted last. The 85 came stock with a MAF burn-off module and it is located in the dash on the passenger side. I have seen them on Ebay and they are going for about what a MAF cost...sorry. On my 86, the burnoff and power relays are located above and behind the battery, on the firewall. One is facing forward and the other is facing to the right. You will either need a 85 FSM or someone with one to help troubleshoot.

sami85L98

Here is the module only for 85 vette, :

http://cgi.ebay.com/ebaymotors/1985-...QQcmdZViewItem

on 86 and later, its located right behind the battery on firewall, usually it will be 2 relays, but on ur 85 u will see only one.

Module is placed on passenger side, under dash behind bread box, loose 4 bolts and take dash completely off, and its right behind bread box, looks exactly shows on ebay ad.

williammackean

The only thing behind the battery on my 85 is a non-connected relay P/N 10032081 for a cooling fan. I have 2 @ P/N 14078915 for the fuel pump and possibly AC, but no P/N 10067925 Relay, prolly Power relay.

This means TPIS was incorrect. Thanks for the Ebay find!

I ordered up a new HD MAF from MAM. For less $$ and increased flow... Why not?

I found a couple other interesting tidbits, courtesy of www.vettetop100.com/1985.htm

Service Bulletin Number: 85C12
Bulletin Sequence Number: 147
Date of Bulletin: 8509
NHTSA Item Number: 81242
Make: CHEVROLET
Model: CORVETTE
Year: 1985
Component: FUEL:CARBURETOR: UNKNOWN TYPE:CLEANER
Summary: PRODUCT EMISSIONS CAMPAIGN (85C12)-MASS AIR FLOW SENSOR-STALL AT IDLE OR NO STA RT-ATTRIBUTED TO CORROSION/DETERIORATION OF AN ELECTRONIC COMPONENT WITHIN "MASS AIR FLOW SENSOR-REPLACE SENSOR WITH P/N 14081243 SENSOR TO CORRECT-1985 CHEVROL ET CORVETTE

and

Service Bulletin Number: VOL 57 #4/11
Bulletin Sequence Number: 155
Date of Bulletin: 8508
NHTSA Item Number: 81259
Make: CHEVROLET
Model: CORVETTE
Year: 1985
Component: FUEL:CARBURETOR: UNKNOWN TYPE:CLEANER
Summary: REVISED MASS AIRFLOW SENSOR BURN-OFF MODULE REMOVAL PROCEDURE- 1985 CHEVROLET CO RVETTE W/5.7L V-8 ENGINE

and (which I have already done)

Service Bulletin Number: 87-100
Bulletin Sequence Number: 051
Date of Bulletin: 8704
NHTSA Item Number: 90919
Make: CHEVROLET
Model: CORVETTE
Year: 1985
Component: FUEL:FUEL INJECTION: UNKNOWN TYPE
Summary: THROTTLE BODY CARBON BUILDUP-1985-87 VEHICLES EXPERIENCE: CODE 34 ON RESTART/HIG H IDLE SPEED/HIGH ACCELERATOR PEDAL EFFORT/ENGINE STALLS AT IDLE OR DURING DECEL ERATION/ROUGH-LOW IDLES-CLEAN THROTTLE-CHECK MINIMUM IDLE SPEED AND READJUST SPE ED AND TPS AS SPECIFIED-ALL 1985-87 CHEVROLET CAMARO/CORVETTE MODELS WITH 5.0L O R 5.7L PFI ENGINE (VIN CODES F AND 8)

jsinga

Hey, if it stalls on both tests maybe it's not the MAF.
IAC or TPS might be causing some of your headache.
Try the minimum idle adjustment and see if you can get it to run.

JS

JAKE

Did you actually examine the MAF to see if the very thin wire(s) are not broken?

You can use the "WITH THE ENGINE IDLING TAP THE MAF WITH THE HANDLE OF A SCREWDRIVER; IF ENGINE STUMBLES - BAD MAF"

Jake

williammackean

If I could get it to idle, I could do that.

JAKE

Okay then just do the visual, unless you've done that already.

Jake

JAKE

You can also borrow a MAF off a buddy's car and if all is well with his, you'll know, definitely. That's what I did.

I have an 86 and borrowed one off a friend's 85. Engine ran like a champ with his. So I bought one off a wrecked Trans Am (many "F" body MAFs will work) from a wrecking yard and it run well until I decided to move up to a 52mm.

Jake

CentralCoaster

I have an 85, but my MAF experience is only with 86-87. Two completely different systems. The MAF is different and in 86 GM replaced the burnoff module with relays and ECM control.

JAKE

Not sure what your point is, but I borrowed my friend's - Alfred Johnson's MAF who lives in Red Oak, TX off his 85 - mounted and plugged it into my 86 and it worked like a charm.

In fact I used it for about a week or so while I was tracking down a replacement for my 86 (his car was down for cylinder head replacement) and it worked fine the entire time.

I was surprised at how many parts that we THINK are specific to Corvettes are actually shared by many other GM vehicles. Take the cruise control servo for example.

But my point is that the original poster should try to borrow one from a friend to see if his MAF is REALLY the cause of his troubles.

Jake

GlennS87

Did you recently add an oil impregnated filter (i.e. K&N)? Sometimes people over oil them a little and the wire of the MAF get coated with the oil which causes erratic MAF operation.