Bill Ball

Put on your thinking caps. During a recent dyno day my Air/Fuel ratios were all over the map, varying from run to run. What could cause this at WOT when the car is open loop? Note the the car seems to be running fine, but tail pipe sampling gave erratic A/F results. I did have new aftermarket cats, but the A/F meter I have mounted ahead of the cats correlated with dyno A/F numbers from the tailpipe.

Initial suspects were fuel filter (95K miles) and/or in-tank fuel pump (155K miles). But the pump is fine. Haven't changed the filter yet, but I got to thinking....the fuel pressure was fine and constant. That points away from the pump or filter. I'll change the filter, as I should have long ago, but I believe this is not the likely cause. What else should I be checking that would affect WOT mixture? MAF and LH are new within the last 2 years and, remember, the car runs fine.

worf928

You write that the A/F ratios vary from run to run. Some comparative variation is normal. Do you have graphs or A/F raw data? Do you have the raw data for ambient conditions?

Tony

When i get home Bill i will DL mine from the data logger from the runs at the drag strip...i havent studied them at all, I just made sure it recorded.

Bill Ball

Dave:

I don't have the raw data. These runs were one right after the other, with just some cool down time in between if the temp gauge read above the midpoint. Ambient conditions were stable and cool. The A/F was very erratic. For example:

Run 1: low RPM 15, dropping progressively to 11.5 at top (not bad)

Run 2: low RPM 12, rising to 15 at high RPM (Uh, that's bad)

Run 3: 12.5 over most of the RPM range (Hmm, well OK, that's better then Run 2)

Run 4: Like run 2 (WTF!)

My in the car A/F meter (on the stock narrowband 02 sensor) didn't go green at all on runs 2 and 4, consistent with the tailpipe pattern. Scared me.

mspiegle

That is very strange. May I ask what wbo2 hardware you are using? I had a problem with my wbo2 unit where it became intermittent and did very strange things like this. After some diagnostics with the vendor, we determined my unit was faulty and I sent it back. I am currently waiting for a replacement.

I noticed when running on Tom's dynapak that most of my runs were consistent except for 2-3 which gave AFRs in the 14s. Were you running on his dynapak by any chance?

worf928

Seeing the curves helps. In this run was the rise to 15 pretty steady?

When was the last time you checked your Idle/WOT switch? If the WOT switch is not activated the LH will continue using the 02 sensor control loop and try to drive the mixture to 14.7. Dyno runs with an A/F mixture at a nice steady 14.7 or thereabouts means the WOT signal isn't getting to the LH.

GlenL

Do the torque numbers confirm moving away from the better power ratios of runs 1 and 3?

Which car?

Bill Ball

Mike: Yes, this was the Dynapak.

Dave: Ah, dang, of course, the WOT switch! I will test it. The Spanner does a one-time pass-fail, and it passes. I will do repeated checking the voltage per the WSM. We did have a couple of steady runs. The we had some erratic ones, some even with the numbers jumping around during the run. I was planning on replacing the throttle switch anyway. It is my original switch converted to work with the 85 throttle body used in the Supermodel SC.

GlenL: This is my 89 with the supercharger. We were dynoing to check the response to the DEVEK x-over. I wasn't paying close attention, as we were at that point just trying to dial in a decent A/F.

Note: when the ratios were all observed to be all over the map, we replaced my FPR with a Kirban RRFPR and the response to increased fuel pressure was still erratic.

I'm thinking Dave is right. Well, hoping anyway. It does need to be ruled out.

Tom. M

Bill, the wot switch may pass the spanner test..but check to make sure it is actually actuating. It is slightly adjustable and you should hear it click when you come off of idle (couple of screws on the side of the throttle assembly. There is a test in the manuals outlining the details...

Good luck..

Tom
89GT

Bill Ball

Thanks, Tom. Yes, the idle part of the switch has an audible click, as it is supposed to. The WOT part, even when working properly, provides no audible feedback. It is supposed to go WOT at 2/3rds to 3/4th throttle. So, you need to go by the test result. The very first time I tested the switch with the Spanner about 2 years ago, it passed idle but failed WOT. I wiggled the connector, etc. and it passed. Subsequently it has pased when I have tested it, but the first failure should have told me it was unreliable. Fortunately with the relocation of the throttle as part of the SC, it is easy to replace.

Tom. M

Just another thought Bill...you have an AT...did you by chance wire in a kickdown bypass switch?...Perhaps the original kickdown bypass switch is not working?...

Just some random thoughts...
BTW..how are you dyno testing the at?...are you disconnecting the bowden cable and kickdown switch..and running in 3rd gear?..

Later,
Tom

Bill Ball

Tom: No kickdown bypass installed. The bowden cable and the kickdown switch were disconnected during the dyno session. All runs in 3rd gear.

mspiegle

Dave makes a damn good point. Just get rid of that bugger and get a new one. Your engine is too important to take a risk.

Bill Ball

No doubt. Funny, I had already planned to replace it and knew it could be a problem, but when finally confronted with A/F, it did not come to mind. That's the neat thing about this group. It more than makes up for me being half brain-dead at times.

worf928

I sense a smoking gun. If the WOT switch is losing connection intermittently during a run the LH is going to be switching from the WOT A/F map to the 02 sensor loop. The first should be driving the A/F to 12.6 while the second should be aiming for 14.7.

EDIT: It may just be a bad connection. Suggest testing the switch on a bench and see if the switch itself closes the connection intermittently. If not then look to wiring, corrosion, etc.

Of course, it could be something else entirely.