I ran a diagnostic program (DIACOM) today on my 85 project vette.

The only thing that jumped out at me was the o2 sensor transition (25-200) and o2 sensor voltage readings (500-700) fluctuated quite a bit. Is that normal or could I need new o2 sensors or MAF sensor?

The car idled high with the TPS at .54 so I adjusted it down to .42 so the engine idled below 1000 (the minimum idle speed screw is already in).

The car actually runs well. I was originally trying to diagnose a fan problem (didn't come on, now [with new relay] comes on too soon) This is the data collected:

PERFORMANCE DATA
------------------------------------------------
Engine Speed................... 700 RPM
Vehicle Speed.................. 0 MPH
Coolant Temperature............ 176 F
Manifold Air Temperature....... 104 F
Throttle Sensor Voltage........ 0.42 VDC
Oxygen Sensor Voltage.......... 668 mV
Oxygen Sensor Transitions...... 151 #
Idle Air Position.............. 32 #
Air Flow Rate.................. 5 GPS
Injector Pulse Width........... 1.6 mS
Spark Advance.................. 0.0 DEG
EGR Duty Cycle................. 0 %
Integrator..................... 125 #
Block Learn Multiplier......... 160 #
Spark Control Counts........... 0 #
Closed Loop Status............. On
Fuel Mixture Status............ Lean
Battery Status................. Norm
Torque Converter Clutch........ Off
Park/Neutral Switch............ On
3rd Gear Switch................ Off
4th Gear Switch................ On
Power Steering Switch.......... Off
Learn Control.................. On
A/C Request.................... No

------------------------------------------------
TROUBLE CODE STATUS
------------------------------------------------
No current trouble codes present

Any feedback would be appreciated.

 

Ok, so whats your question..... ???
check and clean the IAC for the idle issue, also check for any throttle cable bind.
Again: so whats your question????

 

Is it normal for the O2 readings to jump around or should they remain constant? Just thought it strange that they flucuated a lot. I got all this data but don't know what to compare it with.

I've already removed and cleaned the IAC. Cable is good too.

 

YES
it is normal for the O2 readings to move.
This idicates your sensor is working correctly (once it warms up from eht heat )
When the ECM goes into closed loop it reads the O2 figures and adjusts fuel accordingly.

 

YES
it is normal for the O2 readings to move.
This idicates your sensor is working correctly (once it warms up from eht heat )
When the ECM goes into closed loop it reads the O2 figures and adjusts fuel accordingly.

"The car idled high with the TPS at .54 so I adjusted it down to .42 "

Suggest you have problems elewhere ( vac leak?? ) if you have to adjust idle like that.
Did you reset the IAC correctly after cleaning?

 

Hi
I have an 85 also. There seems to be some problems. If you have the "idle screw"in so that there is no butterfly opening then you have a vacuum leak (most probably). You see at the Block learn Multiplier that you are at the max 160 which is a very lean condition. The Integrator keeps the 125 which (if I am understanding this right) is just within control. I guess if you search for vacuum leaks you will find them. Check the hoses and the brittle plastic tubes under the plenum and those to and from the canister. If those are good then spray some start gas or propane over the manifold area when the engine runs and listen to changes in the rpms. If that comes then you have a vacuum leak in that specific area.
The idle rpms is set by the chip. If you have hunting idle it may be a dirty throttlebody or IAC. Clean up the openings for that in the Throttlebody. You can get good access to them if you take off the little aluminium piece screwed to the throttlebody underneath it..
Check also the EGR valve. If it is stuck or the vacuum membrane is bad change it. Vacuum membrane is easy to check. Apply vacuum to the vacuum input and check how long it holds it. It should hold it for about 20 secs

 

If I recall correctly, the 85 ecm sets a fixed 1000 rpm idle speed when scanned.

Readjust the tps back to the normal voltage.

The MAF flow is rather low for 700 rpm (reading only 5 gm/sec). This is what is causing the lean mixture and resulting high blm value. At idle I expect to see the MAF flow roughly equal to 1 gm/sec/100 rpm.

Since the measured airflow from the MAF is less than the (expected) actual airflow, the fuel delivered in open loop being proportional to the measured airflow is less than what is really required resulting in lean running.

When the car goes into closed loop, the high blm of 160 is adding the maximum fuel to compensate for the existing open loop lean condition.

With the MAF flow that low, I'd be looking for a big air leak (air that is bypassing the MAF) or try another MAF.

O2 sensor should show rapid voltage fluxuations when the mixure is near stoichiometric, since it is basically a rich/lean switch.

 

How do you reset the IAC after cleaning? I'm still a newb. I have always suspected a vacuum leak but haven't been able to find it. Lines are definately brittle! Can they be replaced with rubber ones or do I have to use those little skinney things that break when you touch them?


DIACOM Scan was done for 3 reasons (problems):

1) to set TPS without piercing wires with multimeter (doable)

2) to note when ECM told the fan to come on (comes on at 180, can I make it come on later, maybe with an override switch/sensor or do I have to reprogram the ECM?)

3) to try and find out if there is a draw on the electrical system with car off (7 month old battery won't hold charge, cheap battery? abused? came with car).

Car runs good (although I've never checked the gas milage) so all this other data is a real bonus (I think). Now, from the scan data, the injectors and MAF are suspect.

Only #3 wasn't acomplished from scanning. Alternater charges good 13.7 volts. Previous owner said it had "electrical gremlins".

With help from The Forum and a lot of bolt on replacement/upgrade parts, I've kept busy most of the winter and should soon be able to enjoy a dependable topless ride with out breaking the bank! Fingers crossed. Appreciate it!

 

Clean / reset IAC valve
www.geocities.com/jgkov/IACService.html

Bookmark 65ZO1's page , lots of tips and help there
www.geocities.com/jgkov/index.html

Not familiar with '85 settings but 180 does not seem to be stock fan start temp, usually 225;
maybe ECM has been reprogrammed
You can always cut a switch in to turn it off but have to remember to turn it on.
Note , most people are trying to get their C4's to run cooler

"Previous owner said it had "electrical gremlins".
Need to disconnect + cable from battery and put amp meter between cable and battery to see what actual battery drain is.Key off.
Someone else will remind me what the acceptible figure is.
Several known power drain problems in C4's.

"DIACOM Scan was done for 3 reasons"
As you can see , by posting you #'s here, others can see problems you are not aware of.
Great entry level program for diagnostics even if you have to get someone else to decipher it for you

 

160 blm(more than likely max. value) is way lean and past what the ecm can control. (0*degrees spark advance ?) The .02 readings should be jumping, tequila boy has made some good suggestions.

 

The 02 reading should cycle from about .100 to .900 roughly 8 times or more in 10 seconds.

Each time the voltage crosses .450mv, that's considered one count.

When I first bought my 86 I had an ECM problem. The ECM wasn't taking over the timing chores. I discovered this when I connected my timing light and directed it onto the damper. Timing mark wouldn't go past the end of the timing cover tab, which is about 18/20 degrees. Even revving the engine did no good; engine felt sluggish.

I borrowed an ECM from a buddy's 86, installed it and all was well. So I then knew the ECM was my problem, so I bought a re-manufactured one from O'Reilly's for about $100.

That was more than 4 years ago and it's still going strong.

Hope this helps.

Jake

 

Thanks folks! I've been told it could be a vacuum leak and/or a weak fuel pump from the numbers, but if it runs good don't mess with it. In other words, "if it ain't broke, don't fix it".

The electrical problem is the only one that bugs me. The battery seems to be draining when the key is off. Dosen't drain with ground disconnected. This could have started when I installed an aftermarket stereo system. Could a small stereo drain a 7 month old battery? I know it uses some juice, all the time for the clock. I don't think DIACOM can help with this one.

 

Well since you have idle problems like that it "is broke" but it's up to you if you want to fix it or not.

To search for battery drain, with the key Off, hook a DMM in series with the +batt lead and observe current.

Then pull the fuses, one by one till you see a marked drop in current. That isolates the main circuit. From there you need to isolate each component in said circuit till you find the problem.

 

After setting the TPS to .54 the idle raised to about 1200-1400. After talking to the member that loaned me DIACOM, I took his advice and unhooked the battery, then reconnected and drove the car a little so the ECM could "relearn" some values. It now runs and idles perfect.

Also the battery has gained a whole 1.5 volts since parking after driving it 60 miles yesterday. I've been checking it with a volt meter every 12 hours or so, If it goes down again I'll follow 65Z01's advice. I think it just needed to be driven but thanks for the tip!