mytrplseven

Well, lets see....Yup, a continuing post on this issue. Latest finding: ISV was leaking badly when I applied a couple of lbs. of air pressure directly into the throttle body and I could hear the air and saw soap bubbles everywhere. So, new ISV, hoses and clamps, intake and TB gaskets and readjusted idle speed per the manual.

Car really goes at wide open throttle but still hesitates using normal throttle for acceleration.

So now here's the latest list:

Silicone vacuum lines
swapped and adjusted AFM
swapped DME
New TPS, Ignition system (total), FPR, Fuel pump, Fuel filter, clean strainer, rebuilt injectors and new connectors, Verified fuel system pressure and leakdown numbers, verified no vacuum leaks anywhere, New ref sensor and repaired connector.

Car is running lean verified by reading plugs and by forcing richer mixture by disconnecting O2 sensor (acceleration issue disappeared but idle hunted 200 rpm). So that's the bottom line: Car's too lean and I haven't figured out why.

One performance shop near here suggested the issue might be a victim of the ethanol modified fuel and suggested I get some 90 octane marine fuel (no ethanol) and run it with that for a while to see if that sorts it out. After the tank is empty, that's the next step.

The mystery continues.

odonnell

Maybe the O2 sensor is a bad and/or wiring is bad? There's a thing on clark's garage for testing it.

mel_t_vin

Good point...as the WOT maps should take the O2 sensor out of the loop/equation.

Throughout this saga, was the O2 sensor ever spec'd/swapped out?

mytrplseven

Re: O2 sensor. I forgot to mention the O2 sensor and the CAT were replaced with new and had no positive effects. Disconnecting the O2 sensor opens the loop and forces the engine to run richer, which proved my suspicions that the car was running leaner. I looked everywhere to find a picture or diagram of which pins on the O2 sensor (3) were which but to no avail. It's hard to trouble-shoot things with no technical backup.

konakat

So what happens if you drive with the O2 sensor disconnected? Does it still stumble?

odonnell

No chance the wiring could be bad?

Also, is the new sensor soldered in? Dissimilar metals (thermocouples) have a contact potential that varies with temperature and any small temperature difference will produce a galvanic effect, giving a bad reading.

raleighBahn

Check the resistance (at DME) of the NTC temperature sensors. There is one in the AFM, and another that plugs into the block near cylinder 1. If either is defective, it can cause lean (or rich) depending on whether it reads hot or cold. Both of these are used to make air/fuel decisions.

EDIT: you can check AFM sensor by unplugging the plug to the AFM. On the AFM itself, test resistance at terminal 1 and 4. For the temp sensor in the block, do this from DME (battery disconnected), or pull the plug and measure directly.

Control values are in the Turbo Shop Manual, pages 28-23 and 28-28.

mytrplseven

I replaced the AFM with another known good one, but readjusted the wiper arm just to be sure. Didn't help. My car isn't a turbo but I did this check early on and it was good. The temp sensor you referred to: is it the DME sensor on the front of the engine? I installed a new one.

mytrplseven

The O2 sensor is new, Bosch and correct. Just plug and play, no soldering. Car ran perfect when disconnected, except it hunted 200 rpm at idle. I had hoped when I replaced the ISV that it wouldn't but I guess it's out of the loop when O2 disconnected.

V2Rocket

have you messed with the FQS to run it richer?
when i was dialing in my new maf last month the car had no guts at all at part throttle but felt pretty good at WOT...turns out i was way lean, to the tune of WOT at around 15:1 and crusing at 17 or so.

i had to re-scale the injectors and now the WOT is in the mid 12s and cruising is 14s, way more power now...WAY more.

try adding fuel via the switch and see how it runs..maybe theres a clog in the rail??

FRporscheman

Maybe your brand new O2 sensor is bad / out of spec? You could try another one.

But like V2 says, why not just enrich the mixture using the FQS and see how things respond?

raleighBahn

Luckily it is same situation for non-Turbo WRT to temp sensors. Disconnect your battery, connect ohmmeter to DME plugs 6 and 22 (AFM temp sensor). You should get 1.45 - 3.3 k-ohms for 15-30 degree Celsius temperature. The manual notes that interruption on temp sensor causes richer mixture (and short causes lean). For temp sensor 2 (the one plugging in block), use DME terminal 13 and ground and look for same range. Note if your car has been off for some time, both sensors should read pretty close to the same thing, as the coolant temp will be very close to air temp.

For O2, and note that O2 is ignored at WOT (and you believe WOT is fine, so o2 is still suspect), you can at least get car up to running temp and do voltage check. Disconnect the o2 plug in engine bay while running and put voltmeter in terminal 1 (2 and 3 are side by side) and ground. Voltage is .1 volts to 1.0volts. Looks like the graph from Engine manual 28-38 shows ~770+ mV to 1V is going to be rich mixture and ~100mV and lower is lean mixture.

mytrplseven

Thanks, RB. I'll use your tests to rule out or solve my issue. This is the first time I've seen any reference to where which pins on the O2 sensor actually go.

konakat

Sounds like the compensation of the O2 sensor is the source of the stumble. Fixing your lean running issue will fix that. In the mean-time, you should probably continue running with the O2 sensor so that you aren't running too terribly lean.

mytrplseven

Right now the problem is that it is running too lean. When I disconnected the O2 sensor, the engine ran richer and it performed much better, with no hesitation. Only problem, the idle hunted 200 rpm because the ISV was out of the loop. The O2 sensor is a new, proper part number, Bosch unit.