North American Turbocoupe Organization



CEL comes on at idle (pulled codes)
Jeff K Offline
Administrator
#11
Here is the info on idle VAM adjustment.....

First, a little history..... I replaced my VAM around 6 years ago due to a dead spot. The replacement (Rockauto) worked great at cruise and WOT but would set a CM 42 (O2 indicates rich when closed loop expected / adaptive lean limit reached) at idle. I assumed the VAM was indicating too much air flow (too high an output voltage) at idle, telling the PCM to inject too much fuel, exceedign the adaptive limits of the PCM. This was made worse by the 43 psi base fuel pressure I was and still am running to keep up with the higher than stock boost levels. I didnt want to mess with the spring tension, as that often seems to lead to more bad things happening, so I took an old VAM apart to look for another way.

On to the adjustment: FIRST NOTE THAT THIS ONLY EFFECTS VAM VOLTAGE OUTPUT AT IDLE AND VERY LOW THROTTLE OPENINGS AND HAS VIRTUALLY NO EFFECT AT CRUISE AND WOT. How much difference this makes above idle I dont really know, as our PCMs dont allow for real time monitoring of STFT and LTFT, they only tell us when the adaptive lean and rich limits have been reached by turning on the CEL.

The VAM has an air bypass adjustment that bypasses air around the VAM flapper door. More air bypass means less air thru the door and therefore lower air flow reported to the PCM.

Stand in front of the car and look down at the top of the VAM. At the upper right corner is a plug around 1/2" in diameter. The bypass air adjustment is under this plug. To remove this plug drill a small hole around 1/4" deep in the plug, screw in a sheet metal screw, and use a vise grip on the screw head to pull out the plug. Take a long Allen wrench (I seem to remember 3/16", but I might be wrong here) and engage the adjuster at the bottom of the now exposed hole. Turning the Allen clockwise closes off the bypass, causing more air to flow thru the VAM door and output voltage to go up. Turning CCW opens the bypass, causing output voltage (and reported air flow) to drop. My VAM, as it came from Rockauto, had a hot 1000 RPM no load idle voltage of 1.35 V. I adjusted the voltage down to .85 V by turning the bypass CCW to open the bypass and allow more air around the VAM door. After adjustment, just tap the plug back into place. After this adjustment, I had no more CEL at idle and cruise and WOT were not effected in any way.

FYI, to monitor VAM output voltage connect a voltmeter bewteen the BK/W (-) and W/BK (+) wires at the VAM connector.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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BJL Offline
Moderator
#12
thanks jeff! great lil write up!
Brian Larkin
88TC 330,000 miles
Slightly Modified
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JohnL Offline
Senior Member
#13
The problem could be with what Jeff is talking about but there are plenty of causes. Since the O2 is new, we should be able to trust it, as long as nothing happened to it during installation. Jeff already explained about fuel trims. If the engine is running too lean or too rich the ECU will trim as far as it is programmed to allow and then when maxed it will set a code because it thinks there is a problem. You should get a code for rich or lean when this happens. The CEL will go off and on when this condition is intermittent, since it may not be able to trim far enough at idle but may be fine at cruise where an air leak or such is just a drop in the bucket relative to total airflow. I had this exact problem with a leaking EGR valve. Intake piping leaks might also cause it. You can check for leaks and also just block the EGR valve to test.
John Lewis
88 TC, 87 XR, 84 SVO
Currently deciding which is worth my time.
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turbospoolin23 Offline
Senior Member
#14
Thanks for the write up Jeff! I think that should be in the tech articles. Well, I finally got some time today to have a look at the car. Warmed it up, and pulled the codes.


EEC-IV KOEO
DTC:11
System pass, no KOEO fault codes present

EEC-IV CM
DTC:42
1. (cars only) HEGO (HO2S) sensor signal out of range/ always rich
2. No O2S switching detected

EEC-IV KOER
DTC:77
Operator error during dynamic response test/wide open throttle not sensed

During the goose test i floored it and didn't let off until i seen about 4500rpm, but still got the 77 code. I also tested the VAM voltages at hot 1000 RPM idle (BK/W - to W/BK +) and i got 0.63 volts. Stock FPR on the car. I tested for boost leaks again and got the same ones as before. When i only pressurize the turbo, intercooler, and charge pipes i got zero leaks. This is my daily driver so i need to try and fix this ASAP, but i'm not too sure where to go from here.

Here is the cheap boost leak tester i made. I had alomst eveything and only had to buy the cap/collar things from canadian tire for $4.
http://s271.photobucket.com/albums/jj140...G_0121.jpg
http://s271.photobucket.com/albums/jj140...G_0122.jpg
87 TC 5 spd... 17X9 Cobra R's, .60/.63 SVO T3 Turbo, Large NPR, Manual Boost Controller, AEM Wideband, BPV, K&N Cone, Walbro 255, Kirban AFPR, 3G Alternator Conversion, Short Throw Shifter, Stinger 3" Exhaust, Magnaflow Catalytic Converter & Muffler.
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Pete D Offline
Administrator
#15
We will get the adjustment procedure up in the Technical Articles on the web site.

It also appears in the Technical Questions forum on the here on the board:
http://natomessageboard.com/ultimatebb.p...2;t=000024
Pete Dunham


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turbospoolin23 Offline
Senior Member
#16
So why am I getting the codes 42 & 77, and how do I fix these? This is my daily driver and I really need to get this car running by monday.
87 TC 5 spd... 17X9 Cobra R's, .60/.63 SVO T3 Turbo, Large NPR, Manual Boost Controller, AEM Wideband, BPV, K&N Cone, Walbro 255, Kirban AFPR, 3G Alternator Conversion, Short Throw Shifter, Stinger 3" Exhaust, Magnaflow Catalytic Converter & Muffler.
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turbospoolin23 Offline
Senior Member
#17
This problem seemed to fix itself. A/F gauge is working like it always did and no more CEL at idle. I have no idea what was going on...
87 TC 5 spd... 17X9 Cobra R's, .60/.63 SVO T3 Turbo, Large NPR, Manual Boost Controller, AEM Wideband, BPV, K&N Cone, Walbro 255, Kirban AFPR, 3G Alternator Conversion, Short Throw Shifter, Stinger 3" Exhaust, Magnaflow Catalytic Converter & Muffler.
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