I'll try to be thorough and to the point:
Rough and searching idle on 1986 Auto. Pulled code 34. Unplugged MAF. Same rough searching idle but got code 33 instead, which I think I should get with MAF unplugged. Pulled MAF off of a 1985 with no ses codes (known good MAF) and installed on the '86. Disconnected Battery to reset ecm. Same rough and searching idle but code 33 instead of 34 with known good MAF connected. I think I should have gotten no codes with the good MAF conncted or a 34 if there was a low air issue. Instead I get a 33 as if it's not connected. Anyone seen this before?

Put the MAF from the '86 that was throwing code 34 on the '85 and got no ses so I'm pretty sure both MAF sensors are good.
Is my next step to swap the relays behind the battery over to the '86 to verify if they are the problem? Could anyone verify I'm on the right track or missing something?

 

The '85 does not have those relays. It uses a MAF burnoff module instead.

Error code 33 is high MAF output. You can get that if there is a bad connection or broken wire between the MAF and the ECM. You could have a corroded connector pin. The best way to check is to measure the voltage on the MAF output pin.

You can reset the ECM by opening this connector for a second or so (much faster than disconnecting the battery and requires no tools):

 

Thanks for the info. I forgot about the burnoff module in the breadbox of the '85. Checking part numbers I see the maf isn't even the same. Odd I don't get a code with the '86 maf in the '85. I know I have voltage but I will double check the MAF connector for corrossion. I hate to throw parts at stuff but I still think it might be a good idea to change the MAF relays. Car has 160k on it and MAF is newer but the relays are original and covered in nastiness. I noticed both relays have dark grease in the connectors.
Is this factory packed conductive jelly or are they just filthy?
And, on an '86 are the two relays the same or different?
I lost my fsm cd and that makes things a lot harder!

 

I found my manual and if I'm comprehending this correctly, if disconnecting the maf sensor does not change my hunting rough idle condition then it's not the maf and I need to investigate fuel delivery issues. So possible fuel issues causing my idle problems and loss of power are causing the code 34 from the MAF? So I guess I need to leave the sensors alone and check the fuel pressure at the rail, injector ohms, etc.???

 

I don't know what the differences are between years. I do know the '86 ECM has provisions for a Delco MAF with frequency output rather than the DC voltage output that the Bosch MAFs put out. Is the '85 MAF Delco or Bosch?

That's dielectric grease. It keeps the connector pins from corroding. In later years they went to sealed connectors and eliminated the grease.

The two relays are different in '86. In '87 they made them both the same. If you get the connectors swapped in a 1986 car the MAF burnoff won't happen and the MAF will eventually start acting funny (I did that, so I speak from experience).

 

Thanks for the response Cliff. I greatly appreciate the help you have provided. I know the 85 maf is a Bosch unit but I didn't look to see on the '86 but I'm pretty sure it doesn't say Bosch on it so your explanation takes care of that issue.

As a broadcast engineer I deal with dielectric grease a lot but all I've ever used is silicone based stuff that is white or clear. This stuff is real dark and smells like gear oil. I guess my question was if GM used a different compound or is mine contaminated with something?

And lastly, is it safe to say that since disconnecting the maf (and forcing the engine into open loop all the time) doesn't solve my idle and power problem I can rule out the maf?
The code is possibly being set due to a fuel or timing problem that the ecm will not detect?

TPS voltage was a little off so I adjusted it last night. I can easily set it for .54 vdc at idle but never could find a spot that would give me 4.49 vdc at wot without the idle being over .65 vdc. I gave up after a half hour or so. I have .54 at idle and 4.26 at wot. I don't know how this may affect me at road speeds once I get other issues fixed. Adjusting the tps had no effect on my current rough idle/reduced power condition.

 

My original MAF does say Bosch on the bottom.

I have a FCC General Radiotelephone license (used to be First Class back in the days when you had to renew it periodically), though I've never used it. Electronic engineer by trade. The dielectric grease in my car is black. The first time I saw it I started cleaning it out of the connector and then realized what it was so I left it in there.

The old ECMs are pretty stupid. They don't do a very good job of indicating problems. Very basic error reporting. Mostly it's "this signal is high all the time" or "this signal is low all the time".

I'm not sure if unplugging the MAF will prevent the ECM from going into closed loop mode. I recall that the engine has to be running for a certain length of time, the coolant has to be above a threshold value and the O2 sensor has to go above 450 mV.

Setting the TPS at idle to .54 volts is the only adjustment required. I don't know where the WOT voltage requirement came from (it's not in the FSM). I think somebody put that out and it's been repeated forever since then. Another one of those false urban legends. The ECM converts the TPS signal voltage into a percentage of the 5 volt reference voltage. Anything over 70% is considered WOT. That works out to 3.5 volts.

Almost all the analog inputs to the ECM have pullup resistors on them (the O2 sensor is the only exception I can think of offhand -- it's differential). That means that they will read high if the connection between the sensor and the ECM is open.

 

Thanks so much for this information. Glad I don't have to worry about the wot tps voltage. I didn't see anything about that in the fsm either but have read it multiple times on the forum.
I won't worry about the grease anymore either.

I guess pull-up resistors on all the inputs explain why a little corrosion causes high impedance and the sensors read high as if they were unplugged even if the circuit is complete.

In my profession the stability of modern equipment means I usually only have to do routine maintenance and and much less component level work. A lot of new equipment uses tiny surface mount semiconductors so I can't even replace some ic's when they pop. A time is coming when nothing will be field serviceable anymore. I spend most of my time managing stations these days. When I was a contract engineer it was a bit of a chore to carry all my tools, an oscilloscope, frequency generator, spectrum analyzer and other outboard equipment in sugar scoop back stingray. Packing it in there wasn't a big deal but getting it all out was! No trunk access and rising fuel prices forced me into a C4 so I could open the hatch. I have a '99
C5 that I still use as my "work truck" today. The front air dam and radiator supports are shredded from driving through fields to tower sites.

If you had a first class ticket you've been through a few spools of solder. Didn't they go to GROL in 1982?

 

I worked at one job where I was the only electronic engineer -- the other two guys were programmers. I did all the design, PC board layout, prototype assembly, testing and repairs of returned units. I have hand soldered some EXTREMELY small surface mounted parts. We used one fine-pitch connector that had 100 pins with 0.5mm spacing. Those were really hard to do without getting solder bridges between pins.

My first license is dated Feb 2, 1976. Second is Mar 31, 1980. The GROL is dated Nov 20, 1984. As I like to say, I've been around the block a couple of times.

 

If my hand starts shaking a little I can get a solder bridge across 1/16 inch.
You sound like the perfect candidate for the job of figuring out the fix for first gen C5 vette EBCM modules. The later units usually have cold solder joints or the surface mount relay goes bad but nobody can seem to figure out the early ones and it's virtually impossible to find a replacement. I've never seen the circuit board but it must be something like a multi pin cpld that is stumping those that try.

As for the vette, I'll continue in my attempt to diagnose what is going on. I'll probably end up making a ALDL cable and downloading a logging program so I can see actual numbers and not just error codes.
Thanks for all the help so far.

 

You'll probaly need a dedicated (that means somebody else made it just for these cars) cable. The ECM starts out at 160 baud and then switches to 8192 baud if it gets the right inputs. Not exactly standard. The easiest way to go is to buy a USB to ALDL cable. Roughly $75, depending on who you buy it from. I just found out that there is also a Bluetooth to ALDL adapter, if you like to go wireless.

It's interesting to me how the 160 baud came about. The ECM has 16 subroutines and it executes them 10 times in 1 second: 16 x 10 = 160 times/second = 160 baud. It sends out 1 byte each time it executes a subroutine as part of the subroutine dispatch routine.