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Help! ECM Code 34, 36 and 42

bczee

Well-known member
Joined
Dec 27, 2007
Messages
52
Location
Concord, CA
Corvette
1988 White Coupe
I picked up a 88' C4 Coupe, L98/A4

I need to resolve the ECM Code of the following, any suggestion or procedures (or where to find them)

Code 34 - MAF Sensor Low
Code 36 - Ingition System Circuit Error or Transaxle Shift problem. or MAF Sensor Burn off failure
Code 42 - Electronic Spark Timing

I plan on replacing the MAF Relays and clean the MAF and connections for Code 34 and maybe replace the MAP.

Need help to understand Code 36 and 42.

Thanks for the help in advanced.
 
bczee, a good investment would be the GM service manual. It explains in detail why a fault code is set, and provides a flow diagram of tests to pinpoint the failing part. If you search this site by using "code 34", "code 36", or "code 42", you will find a wealth of infomation.
Code 34 indicates ECM has seen low air flow. This could be caused by an unstable idle.
Code 36 indicates ECM failed to see the MAF burn off (cleans MAF wire). Could be a poor connection. A faulty MAF should not be considered with a code 36.
Code 42 indicates that the ECM saw voltage on the EST line (white wire between ecm and ignition module) when it should have been grounded by the ignition module. Could be poor connection, a faulty ignition module (in the distributor), or a defective ECM.
 
TedC

Thanks. I just picked up this '88 today.. have not had time to invest in a manual (other than a Chilton one that came with the vette).

Need to resolve these failure in order to have it pass the smog test..

I was planning on cleaning all of the connections and the MAF and Throttle body. Change the MAF Relay (I so happen to have two relay that I think are the correct part numbers) That are left over from my LS1 swap into my Chevelle.

I think I will consider replacing the Ignition Module and MAP sensor(as that seem to be related in the code 34??)

Any other suggestion for the time being ?

Thanks much.. I am a newbie to the C4 but not to working on cars.
 
I offered a thought on the original thread. You're on a great start to working on these cars. An engine is an engine and the basics must be right, whether carbureted or ECM controlled. I'd do a basic tune-up, looking at fuel and spark, like any other car, including setting the timing and checking vacuum. When, and only when, the basic items are okay, head for ECM country.

The included manual might be good for soaking up spillage; get the factory manual at your earliest!

You'll find three 'new' tools helpful and not too pricey. 1) a fuel pressure gauge, 2) a digital VOM, and 3) a way to pull codes, whether you opt for a paper clip or a more sophisticated tool. Cleaning connectors and ensuring that they and the associated wiring is in good shape, on an electrically controlled car is very wise, especially as many voltages and currents are low.

Enjoy :w
 
Congrats on the purchase of the '88.

Try these links out, maybe they can help until you get a FSM.

http://www.corvettefever.com/techar...et_corvette_trouble_codes_problems/index.html

Click on ECM CODES

http://www.corvettebuyers.com/c4vettes/indepth_launch.htm

Have you checked to make sure that the timing control wire is secure?

153_0303_code_18_z.jpg
 
WhalePirot (and all)...

thanks for the input and suggestions, The car does run decent, oil pressure gauge holds around 55-65psi (I'm suprised). Suspension seem to be in good condition for 147k on the clock. Does need a lot of TLC to bring it up to a Daily Driver standar.

Its just throwing the codes. So I plan on doing a Major tune up, clean as many parts and compones as I can. Clean and reseat all of the connection associated with the problem areas. Replace what is needed (but will verify 1st if I can).

I have WinALDL on my Laptop and a MaStach cable. It should be a little more helpful than just the code reader. If I can get over to my friends house (used to be a Snap-On Dealer), He has a better scanner if the WinALDL doesn't work out.

I have the two MAF Relay, so I may as well install them.

I'll keep everyone posted on my progress to become a Corvette guy now..(add this to being a Chevelle (LS1 Conversion), Nova, Z28 Camaro, Fiero and VAN guy now LOL). As you can see I am used to doing a little work since I did my ground up Resto and conversion of the Chevelle, Updated and swapped in a 350 in my Z28 in my garage. I think it is time to get a lift !
 
boostedmaxPSI.. thanks.. very good information ... as soon as I can get around to checking everything and following this artical.. hopefully it will resolve the codes..

bossvette - cool.. once i get my problems resolved.. I will ping in there too :D I got the truck for free from my cousin after my uncle passed away.. The truck was sitting in the garage since '75 due to the brake master going out.., it sat for a few years and then he passed away.. I pulled the truck out 3 years ago when I have to give up the garage space and now sits in my side yard under a tent.. I have since make sure that the engine turns over and have rebuilt the Fuel system. Once I convert the Electrical to 12 volts, i will try and start it up and then continue to fix the brake and what ever else needs fixing. I plan to leave it stock and the interior is all original and in very good shape. The body has some rust but not to bad. There is a Flat bed on it as it was used on my other uncles farm for a few years.
 
Well.. I Sea Foamed the engine to day.... clean up the engine bay a bit.. swapped in the Relays, cleaned out the Air Box and the tubes.

Sprayed the MAF with some MAF cleaner, use some Electrical cleaner on the connectors and started it back up.. well Code 34 and 42 are gone (for now?) but the Code 36 is still there.. noticed that the connector for the MAFwas a bit out of shape.. wires don't seem to be melted together.. I will try and replace it later, but will check to see if they are shorting together or not tomorrow.

While I was cleaning my Air box.. I found that the Air Filter was soaking wet !.. What have you guys been doing to keep the water out of the air filter box? (a performance box with K&N might be the plan).
 
Well looks like my efforts have resulted in clearing up all of the codes !

I also replaced the Throttle Body gasket, reseated some vacuum tubes and hoses and checked and seated some more connections.

The Throttle body was pertty dirty from carbon.. I sprayed it down with some more cleaner. I think I will also have to replace the Injector O-rings.. I think they are leaking some too. But I think it is running well enough to pass the smog so.. I will see about it this or next week after I change the oil, filters, spark plug and the cap/rotor.

Later down the road i will need to start swapping out some of the other hoses and other connector/wires as they are showing there age. But next will be the passenger head light (gears and motor ?) and the passenger window (regulator ?)
 
I am not a huge fan of the K&N, as a large pile of paper filters can be had for the same $$ and they flow as well when new. Understand, I have 3 or 4 K&Ns on various cars as I liked the green aspect of just cleaning, not discarding. I just don't like the performance hype and the cost.

Be careful about the SMOG rules; you don't want the car nailed as a gross polluter! Some places down here will do a pretest.

While the injectors' rings may be shot, three tanks of high quality detergent gasoline will clean them up and you just might continue the streak of luck you're on.

I expect that I am among many that are happy that your Corvette-world entry is going well. :w
 
WhalePirot

Thanks for your input... I have used K&N without any problem and yes, I understand that some have had problems and other don't think that they Perform or is worth the cost. I guess I am just one of those old guys.. if it ain't broke, then don't fix it.. and if it works for me. .then I will keep using a produce until I find it is not for me..

Yes... I am very carefull about getting my cars to be clean before any test... I have had problem with a few in the past. Pre-Test are good. I am currently running a tank and can of Sea Foam through it now and will continue to run some good fuel prior to a smog test.

I have been fixing a few more problem area, found some bad vacuum hose and repaced some of them. Ran a bit smoother now and some better throttle response now.

Had a bit of a problem last night. After a 30 mile highway trip.. I got home and put it into netural.. and the idle was racing (1200-1400 RPM).. I let it race for a bit to see if it would come down, but it did not. I then hit the throttle a few time, but it remained at a high idle.. I have not had a chance from last night to check anything or see if it is still at a high idle.. any one have any input or suggestion as to what is causing this condition or fix for it ? Is this a common problem?
 
IAC sticking? EGR sticking? TPS not sending correct voltage (.54 volts at idle)? Vacuum leak? That idle is high. I've not see this on either a 1988 or 1989 L98. Any chance water got into the TPS and is shorting it out (based on your air filter issue)?
 
TedC

I have been researching and I think I will try and set low idle for the IAC, and TPS, but looking for the correct procedures to make sure I doing it right.. I read a few thread instructing of how to do it..

Anyone have a good write up or web page ?

thanks
 
How to Adjust your Early C4 TPS and Idle Speed
by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. New 6-15-00

This tech paper will discuss the procedure for correct adjustment of the Minimum Idle Speed and for adjustment of the Throttle Position Switch (TPS) on the early C4 Corvette TPI systems. These steps apply specifically to the 1985 model year, and in general to other years. Later model years do not have adjustable TPS's.

General
Idle speed and off-idle throttle response on the early TPI systems is determined by correct adjustment of the minimum idle speed screw combined with a correct setting of the TPS. I've seen many of these cars that have had their idle speed "corrected" by well-intentioned mechanics and owners by simply screwing the minimum idle speed screw in a few turns. This really messes up the settings, and will not make your car perform properly. Doing a correct setup of the TPS is one of the easiest ways to make your car feel and respond better. To maximize the benefit of this procedure, I recommend that you first remove your Throttle Body (TB), disassemble it (it's incredible easy - there are a total of about 5 pieces in it…), clean the TB up really good with some spray carb cleaner, and put it back together. A nice clean TB will really put an edge on the performance improvement you will get by doing this procedure.

The Service Manual has instructions for doing these operations, but the directions are scattered through several sections of the Manual. Here is the complete, step-by-step process for doing this (not including TB rebuild). All specs and steps are taken directly from the Manual (all 3 different sections), and this process is absolutely correct.

Tools & Equipment
You will need the following tools and equipment:

1. A set of Torx wrenches. You can buy a complete set in a nice, genuine plastic pouch at Sears.
2. A good digital voltmeter that will read voltages less than 1 volt.
3. A paper clip.
4. A small screwdriver.

Procedure
There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC.

You will also be playing with the diagnostic connector under the dash. Remove the cover (if it's still in place). Bend your paper clip into a "U" shape. You will be playing with the two top right hand terminals ("A" and "B") in the connector.

First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It's located on the driver's side of the TB. Remove this plug if it's there.
With the IAC connected and the ignition "OFF," stick the paper clip into the diagnostic connector from "A" to "B." This grounds the diagnostic lead.
Turn the ignition to the "ON" position without starting the engine. Wait 30 seconds.
Now, with the ignition still in the "ON" position, disconnect the IAC connector at the IAC.
Remove the paper clip from the diagnostic connector.
Start the engine and allow it to reach normal operating temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while.
If your car is an automatic, set the parking brake and put the transmission in "DRIVE." If your car is a manual, leave it in neutral.
Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
Shut off the engine and re-connect the IAC.
That's it for idle speed. Now on to the TPS.

There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.

Turn the ignition to the "ON" position without starting the engine.
Loosen the TPS Torx adjustment screws.
Set your volt meter to a low scale DC volt setting that will accurately read less than 1 volt.
Measure the voltage between the two top TPS wires.
Adjust the TPS by rotating its position until you get a reading of .54 volts.
Tighten the Torx screws and recheck the voltage. Re-adjust if necessary to make sure voltage is right at .54.
Turn the ignition "OFF."
You are now in perfect adjustment on idle speed and TPS output. Start the engine. It may take a few seconds for the car to "catch on" to its new settings.
 
TedC...

Your good.. that was quick...

thanks..
 
Ok.. I did the low idle adjustment for the IAC and all seem ok tonight and after a 65 mile drive.

Thanks for the information and procedure.. I still need to check the TPS.. but that will get done this weekend..
 
Excellent news... Keep us posted... I would also suggest replacing the oxygen sensor. This way the ECM is receiving accurate mixture infomation to make fine adjustments (like IAC step adjustments). The old single wire (non-heated) design used on L98s age slowly and do not respond as quick as a new sensor. Recommendation is to replace between 30-50K. On my 1988 which only had 50K, the difference was night and day. Sensor cost is $20-25, and if you use the special oxygen socket, it is pretty easy to replace. I'm waiting for nice day to replace on my 1989.
 
TedC. yes.. I have not gotten around to doing a complete tune up yet.. but this weekend will be a Oil/Filter change, the Cap/Rotor with Spark Plugs.. and maybe the O2.. it is on my list of 2do's... also have to do the Trans Filter/Gasket.. as there is some leakage from the Trans at this time.. it has been oiling down the under side and burning on the rear Y-pipe.

Other items on the 2do list.. right headlight motor/gear, right door window Regulator(?) and window seals, driver seat belt latch, both wiper arms and replace missing left hood latch.. and just clean the car inside and out. Longer term project will be to restore the interior, but I am in no rush and have the time to search out cheap bargins. I know that I got this vette on the cheap.. and expect to spend some cash on it.. but I figure that I will soon have it in the mid Blue Book range for way less that I am spending on it.

But the good news is that it seem to be running very well now!:D
 

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