IROCVert88

I have been trying to figure this thing out for a while now and am about to give up. Heres the deal, I am getting a code 34, and it will trigger the check engine light, only at idle. If I sit at idle too long (i.e. stop light) but if I hold the brake and press on the gas a bit too keep the rpm's up the light wont come on. The car runs fine with the maf unplugged and seems to idle fine before the light comes on and after. Before the light comes on the rpms dip to 500 then back up and then the light comes on. I am at my wits end with this thing, I just wanna know what will fix it!

CaptPicardsZ28

Code 34 is an MAF I believe. It could be your IAC valve needs cleaning or replacing. When I replaced my IAC valve I got a 33 code which is MAP. I had to erase it from the ECM. Idling problems are usually from the IAC but it could be a vacuum issue as well. Try resetting the ECM. Hope this helps.

IROCVert88

It doesnt really have an idle problem except for its a little low. But the check engine light will only set if the car is at an idle.

BTW, I know 34 is a MAF voltage too low, and 33 is MAF voltage too high. Does anyone have a troubleshooting tree for code 34?

IROCVert88

would anyone have a troubleshooting tree for code 34? Anything to get this figured out.

Morley

That is a false code 34, there was a TSB on it years ago. You need the newer, updated Prom chip (about $45 from GM).
It triggers the check eng light while at idle but has no effects on idle. This updated prom also resets the idle to 650 in gear, cures the cold drive away stumble, torque converter lock/unlock hunting... pretty long list of problems it addressed.

IROCVert88

How do I check and see if I already have the chip? Just incase.

Jim85IROC

iTrader: (2)

The MAF power relay can cause a code 34 too. If the contacts get dirty the voltage drops through it.

An old battery can even cause this. Right after startup the voltage is low from the weak battery and can throw the code.

IROCVert88

the battery puts out good voltage, and with the car running its well over 14v.

Where would the power relay be located? I looked at the far left side of the firewall and there was only one relay there, shouldnt there be 3? Where else would the relay be on a 85?

Morley

There is no relay for an 85 TPI, it used a MAS module to handle power and burn-off functions. The MAS is the little box that is piggybacked to the ECM and is an expensive little bugger to get from GM (on the order of $700-$800) if you can still get one from them.

Jim85IROC

iTrader: (2)

there sure as hell is a power relay for the 85 MAF. There's just no BURNOFF relay. The power relay is on the firewall between the fender and brake booster.

Anyway, having 14 volts when the car is running doesn't mean dick about your battery. Your alternator is providing that voltage, not the battery. If your battery drains down to 11-ish volts during startup, it can cause the MAF to throw a code before the alternator starts charging.

IROCVert88

the check engine light doesnt pop on though until the car has been running, and the alternator starts charging almost right away after start up. So I dont think its a battery voltage issue. I am going to give the prom a try, atleast see if the new one is in there. So how can I tell if the new one is in?

Morley

There is only one relay there and that is the fuel pump relay. The MAS module is power and burn off.
Check out the article on changing the 870 to 165, you add two relays to replace the module, one for burn off the other for MAF power.

Morley

The updated prom chip will say:
HLM
3800
on the silver sticker on it.

Jim85IROC

iTrader: (2)

You want me to take a picture of my harness, or will the schematic in the Helm manual suffice?

Morley

Do whatever you think you need to. If it is in the manual it is WRONG, plain and simple. I have owned my 85 since it was new and have done ALL of the maintenance/ mods myself. If nothing else, I know that car, every single inch of it.

Did you even look up the article on swapping to the 165 ECM???

Here is the picture of the one and only relay on the driver side, Fuel pump relay http://www.eecis.udel.edu/~davis/z28...umprelay85.jpg

Here, I'll make it easier, this is the link to the swap from 870 to 165 http://www.eecis.udel.edu/~davis/z28/ecm_swap/

Oh, and I have done this swap myself, you ADD a burn off and power relay (2 relays added) by splicing them into the MAS module harness.

I'm not trying to flame here, merely to educate.

Jim85IROC

iTrader: (2)

I've done the swap too. My 85 has a 165 in it now. With the original harness I had 2 relays on that bracket.

OK, I'm eating crow for now because the more I dig the more apparent it becomes that I'm wrong. I'm digging out that old harness tonight before I keep up the fight.

Morley

I'm not trying to make you look bad, but I don't want him changing that relay and thinking "the power relay didn't work, must be the MAF" and have that not fix it either.

If you had 2 relays on that bracket...I have no clue what the second one might have been because my bracket is set up with room for only one relay and the ESC module. Possibly a relay from something the previous owner had installed.

BTW is still have my 870 harness installed in the car (I did the repin and splice method instead of new harness) and the lone relay on that side is fuel pump. I put my 2 "new" relays on the bracket that the MAS module was mounted to. That saved having to run more wires out to the engine bay and through the firewall.

Jim85IROC

iTrader: (2)

My bracket was identical to the one that came with my 87 harness. It even had the holes for all 3 relays in it. I didn't get time to dig through my boxes to find the harness, but I'm going to when I get a chance. I'll either take pictures of the 2 relays, or I'll eat crow and admit that I was dead wrong. I do remember that the harness was completely unmodified (well, at least until I wired a heated o2 sensor in), so I'll just have to see what it's got.

You guys know me well enough to know that I don't go around spreading false information, and I'll never say something unless I'm completely sure of it. That's why this surprised me so much. I'm still sure of it, even though obviously I was wrong. It pisses me off BIGTIME when I goof up.

Morley

If you want I could scan the complete 85 engine wiring schematic and E mail it to you. It shows the power and burn off wires coming from the MAS module. The schematic is a little pen marked from all my years of tracing wires durng trouble shooting. The schematic is from a very nice Hygrade manual I got years ago. It covers up to 1987 year by year and engine by engine, no generic schematics.

IROCVert88

Well it looks like I have the old chip in the ECM, I couldnt see the whole silver sticker but I saw HLK and Morley said it would say HLM? I have one question now, how do I change out the chip? I didnt really see any clips or naything holding it in so whats the trick to taking that thing out?

Morley

When you order your new chip you SHOULD get a "chip puller" with it (I did when I got mine). It is basically a U shaped piece of spring steel with a "hook" bent on each end to go under the lip of the plastic carrier that the chip comes in. You hook the puller under the lips and gently rock the chip end to end and pull up (also gently). Remember which way the chip faced (by the little notch in the chip) in the ECM and install the new one in the same direction, making sure it is sitting even and all the way down.
The new chip will also come mounted in a new plastic carrier, so you don't have to fool with trying to get the chip out of it and back in.

And yes, HLK is the origonal, defective factory chip. I have my old one sitting here.

Jim85IROC

iTrader: (2)

Thanks, but I have my helm manual at home that I can check out and compare to my 87 manual.