OK, guys, I really need some help diagnosing this. About 2 weeks ago, my SES came on. Using a jumper wire, I pulled a "code 34". I cleared it, and after disconnecting the MAF, it threw a "33", indicating that the wiring was OK (had there been a short or loose connection, I believe it would have still thrown a "34"). Next, I switched the two relays for the MAF as per Vader's picture (thanks Vader!), and a "34" still came up. I also checked for any vacuum leaks, but came up empty handed. I finally replaced the MAF with a new one, cleared the codes, and, still threw a code "34"!
The catch is it only does it after sitting long enough to cool off (overnight). If I start it, the SES will come on, but then if I let it warm up for a few minutes, shut it off for a few seconds, and restart it, the light stays off. Anybody with any ideas on this? Any help would be greatly appreciated, as this is getting very frustrating! Thanks, and sorry for the novel.

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'87 IROC Z28
TPI 305
5-speed
3 1/2" exhaust w/ Flowmaster
Clarion, Kenwood, Eclipse, Polk, MTX, Autotek 1600 watt stereo system
Grant wheel

 

I was setting a code 34 for about a year until I found a little crack in one of the thin plastic vacuum lines. I fixed that line and the code went away.

 

I'll go back and triple check all the possible spots a vacuum leak might develop. Which of your lines was cracked? Anybody find a successful method of tracking down a leak aside from dousing them with carb cleaner & looking for a rise in rpm?

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'87 IROC Z28
TPI 305
5-speed
3 1/2" exhaust w/ Flowmaster
Clarion, Kenwood, Eclipse, Polk, MTX, Autotek 1600 watt stereo system
Grant wheel

 

When you swapped relays, you switched the MAF power relay with the MAF burn-off relay, yes? Also, did you reset the codes after the swap? When you say you replaced the MAF, you mean the sensor itself? The GM dealer has service buletins on these codes. They usually just replace the relays and the codes go away. The sensor is expensive. I had code 34 intermitant for a while, swapped relays and then I got code 36 (i think), so I just bought a new relay at 11 bucks and no codes since then.

Hope this helps.

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Best ET 14.413 @95.57 without
pulling valve covers or manifolds.
Also with stock 2.77 rear end!!!

 

The line that was cracked for me was the tube that goes from under the front of the intake plenum on the driver side, back to the vacuum switch on the coil bracket between the coil and the distributor.

Most likely, I cracked it myself when I took the plunum off to grind away the tabs that restrict airflow.

I've often wondered about finding vacuum leaks. One thing I have been thinking recently is maybe you could generate a vacuum with some kind pump, and (with the car off) connect it to say, the cruise control or something and see if any vacuum leaks out. Is there a tool that does this? I'd really like to thoroughly check my vacuum accessories.

Right now I have my cruise disconnected because I'm suspicious of a leak.

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Michael Bangert
mbanger@iupui.edu
'88 5.7 IROC
'99 CBR1100XX
'00 Silverado LT

[This message has been edited by tregnab (edited March 24, 2001).]

 

As far as swapping the relays, yes I switched the MAF power and burn-off relays, and yes I did clear the codes afterwards. And, unfortunately, it still threw a "34" at me. Also, yes I actually bought & installed a new MAF sensor ($ ouch) after I had swapped the relays, checked for vacuum leaks, and unplugged the old MAF sensor to see if it threw a "33" or "34" (clearing the codes in between each of these steps). Today I will be going over every inch of vacuum line I can find to see if there might be a leak I missed before. The fact that it only gives me trouble after it has been sitting for a while makes me think it is related to a vacuum leak somewhere. Any other ideas will be very much appreciated! Thanks!

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'87 IROC Z28
TPI 305
5-speed
3 1/2" exhaust w/ Flowmaster
Clarion, Kenwood, Eclipse, Polk, MTX, Autotek 1600 watt stereo system
Grant wheel

 

IROC,

While it is important to inspect the vacuum lines, you should also be aware that leaks in the intake ducts can cause the same DTC. The plastic resonator boxes and rubber bellows need to be inspected for cracks, splits, and poor (leaky) connections.

The breather tube for the PCV system needs to be connected at the throttle body fitting, and the PCV system must not be leaking. This means leaky rocker cover and timing cover gaskets, intake "china wall" seals, and oil pan gaskets umst be leak free. Using a breather-type oil filler cap will set a '34' right away.

The cover plate on the throttle body needs to be sealed (the one that reads "Tuned Port Injection" on its top), and the TB itself must be sealed to the plenum. Essentially, the engine case must be sealed vacuum tight to prevent leaking air from skewing the MAF readings.

The throttle position sensor needs to be properly adjusted so that the ECM expects the correct amount of air at a given RPM.

All electrical connections in the system from the sensor to the relays to the ECM must be clean and tight.

A modified MAF can also cause incorrect readings due to the lack of equalized air flow through the sampling tube in the sensor body. The screens must be in place if you are getting this DTC.

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Later,
Vader
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"I'm'a do Things My Way - It's My way or the Highway."
Adobe Acrobat Reader

 

Yes, the plenum, runners, and TB absolutely need to be sealed up too. Wouldn't hurt to tighten down those bolts while you're looking around. I had just one of those slightly loose after some tinkering one day, and the car ran like crap until I found it.

If I'd just leave the hood closed it would probably never have any problems

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Michael Bangert
mbanger@iupui.edu
'88 5.7 IROC
'99 CBR1100XX
'00 Silverado LT

 

Just like to add that I sometimes can cause a code 34 on my IROC if I hold the throttle slighly open on startup. Probably means more tinkering for me too.


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Michael Bangert
mbanger@iupui.edu
'88 5.7 IROC
'99 CBR1100XX
'00 Silverado LT

 

Does the engine idle smoothly?

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Michael Bangert
mbanger@iupui.edu
'88 5.7 IROC
'99 CBR1100XX
'00 Silverado LT

[This message has been edited by tregnab (edited March 24, 2001).]

 

Hey guys,
Just for an update, I may have found the problem. I went through and check the T.B., plenum, runners, and the T.B. cover plate and all were fine. Same for the air ducting and bellows, PCV, rocker covers, & timing chain cover. It seems to be air-tight. Then I went back to the electrical. TPS is ok (.53 @ idle, 4.8 @ WOT). The connections for the relays were clean, but the was a small section of two adjacent wires that were bare going to the connector for the power relay. If they touched, the relay started buzzing! It would have been easy for these to touch with the car going over bumps or whatever. The actual wires were still fine, so I put some heatshrink on them, and reinstalled the relays, cleared the codes one more time, and went for a drive. The car was cold (and this is when it always sets the "34"), and started and drove with no problems, and no trouble codes! Hopefully, this solved my problem. I'll post back in a day or two and let you guys know.
A sincere thanks to everyone for all the help!

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'87 IROC Z28
TPI 305
5-speed
3 1/2" exhaust w/ Flowmaster
Clarion, Kenwood, Eclipse, Polk, MTX, Autotek 1600 watt stereo system
Grant wheel

[This message has been edited by IROCketZ28 (edited March 25, 2001).]

 

cool, glad you found it.

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Michael Bangert
mbanger@iupui.edu
'88 5.7 IROC
'99 CBR1100XX
'00 Silverado LT

 

Any luck yet?
I just replaced my relays, and STILL get 34. One interesting note is that somehow there was a chip missing off my MAF which I threw some duct tape over.

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1980 Camaro, '88 Police 350 engine, Headers, full exhaust, completely redone, my baby!
1988 Iroc-Z L98, MSD 6a, Accel coil, K+N's, Airfoil, Coolant Bypass, Flowmaster, Gears, Late model Bose, I like the 80 better

 

Good news! It's been 3 days now, and it hasn't given me any problems. Looks like the problem has been resolved! Thanks again to all!
Steve R: try the following. It may help to narrow down the source of your "34"...
1. Clear codes
2. Disconnect wiring harness plug from MAF
3. Start the car, and you should see the SES come on
4. Shut off, and pull code
5. If it still threw a "34", then go over all the wiring and vacuum items listed in the above posts.
6. If it throws a "33", then it's time to replace the MAF sensor.
These steps were from the diagnostic software from a reputable local shop that I have access to the computers at. I only have minimal experience with this procedure, though, as I have only done it on my own IROC to try to solve my "34" problem. I am always open to any contrary opinions. Try it, and let us know the results.

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'87 IROC Z28
TPI 305
5-speed
3 1/2" exhaust w/ Flowmaster
Clarion, Kenwood, Eclipse, Polk, MTX, Autotek 1600 watt stereo system
Grant wheel