gasmanstl

John:

Perhaps I could have done better with a longer, more nicely composed disclosure of the nature of the car...this auto is, in fact, the silver 86 S which is seen in Mark Robinson's videos on his "928turbosports.com" website. This vehicle was the developmental model for the rear-turbo kit, which he now sells. He abruptly abandoned this car when he found a nice garage-queen, and sold the car to purchase her. At the time, he had removed the kit to send it off & have it mass-produced. He was just getting the first production kits, and I was asked if I wanted the car with or without a turbo...I was going to murf-blow it anyway, and I thought the turbo option was easier and had certain advantages.

I have it on Mark's history that the developmental kit ran for 7000 miles, very well, with the injection working correctly, WITH the 30 lb injectors, the mixture right, and the wideband O2 meter demonstrating that the computer was demonstrating "closed-loop sampling." (If I screw up on nomenclature, please just correct me, I have learned everything I know about porsche EFI from this car, fairly recently. I am still a newbie, in this regard, for the most part.)

So, for me to pick up the car, Mark installed the turbo kit. He was just finishing it as I arrived, and as far as I have witnessed, ever since this production kit was installed, the car has demonstrated the problem I am trying to solve. Mark's thought, after replacing the O2 sensor initially, which didn't work, was that the MAF was bad.

I then drove the car home running rich (but not too awful - getting 16 - 17 mpg) - and successfully got the mixture correct AND the computer to demonstrate closed-loop sampling - by turning the fuel pressure down...to 10 PSI...way too low. But the car ran fine, mixture was right, and the computer was working the right way, according to my understanding...while I awaited the MAF.

He then obtained a rebuilt unit from you, which is now installed. Now, the info on this thread completes the story. As far as I can see, the problem is the same...the only thing I am unsure about is if I can even get the mixture right again by turning down the fuel pressure. I didn't try that last night, I thought I'd post the above results first.

Sorry for the confusion.

John Speake

OK, now we are on track....

It may be that the car was just about running with a badly aged , weak running MAF. With a rebuilt MAF you are just to rich for the loop to cope, with those 30# injectors.

The other issue now that Flyingdog has mentioned is that your O2 sensor may be U/S due to contamination from the very rich running.

Try disconnecting the O2 sensor, use the original 85/86 LH ECU, and adjusting the CO pot again.

gasmanstl

OK - but will the AEM meter still work with sensor disco'ed from brain? Or would it even be worth anything with a slopped-up O2 sensor? I just wonder what my endpoint will be when I'm adjusting the CO pot, with no mixture info (Remember, I have no CO meter.)

John Speake

You can of course try with the O2 sensor connected. See if its readings are plausible. e.g. if mixture very rich reading will be 200mV or so. Increasing as the mixture weakens.

gasmanstl

Really? So 700 - 800mv is a *lean* number?

This totally tells me the O2 sensor cannot be working right. I basically understood that if the thing was generating a number, it was working, so assumed it was OK. Mine would start out, engine cold, at about 300 mv...warm up, get to about 800. It was so rich, you could smell the gas baaaad...

I think I'm going to buy a CO meter...I see a few online. This should be more definitive, and I hate ambiguity... I'll keep you posted...

gasmanstl

Last night, I bought a Gunson Gastester CO meter on ebay. If the guy ships it like he oughtta, I should have it in a couple days.

So here is my plan - this is necessary because my TPS is never necessarily working right. I plan on jumping the idle switch terminals in the harness, with the plug undone. This means the WOT cannot be seen as shorted, and the idle switch will be seen as closed - thus ensuring I am in the idle map. Then, I'll turn that pot back in a couple of turns (clockwise) and, using the meter, attempt to adjust the exhaust gas for 1% CO before the cat. Is there anything wrong with this reasoning?

I have a new O2 probe, to install when the mixture is adjusted.

Thanks!

John Speake

Seems a good plan of action, let us know how you get on.

gasmanstl

OK. Had a nice day with the Gunson Gastester (not.) It is, btw, an extremely finicky device, which is hard to calibrate, and drifts a lot. I don't recommend it.

Anyway, here's what happened: with things "as they were" (old O2 sensor, fuel pressure 24 psi) - CO reading was mid-7% range. Wow. This is with the mixture pot on the MAF turned all the way out, that is, R between terminal 6 and body of MAF is near zero (never got below about 0.3 ohms). If I un-jumped the idle switch terminals, simulating tooling down the road under very light throttle, the reading was about 15%.

Sooooo...I turned the fuel pressure way down (like 4 psi) - then replaced the oxygen sensor with the oft-recommended universal part spec'ed for a 1989 Ford Taurus, soldering it in nicely, and also soldering into the black wire a T-wire with a spade connector, to hook up to the AEM wideband meter, and to measure the voltage of the sensor with a voltmeter. Started her up:

After warming engine, it was clear that the mixture was lots better, just by the smell of the exhaust. Wideband O2 meter was still reading RICH one bar, and the measured voltage was about anywhere from 30 - 50 mV! A veeeery rich reading?? At this point, the AEM demonstrated intermittent sampling (lights would go up to the one-bar-rich bar, and then back to zero, about every second). This is my understanding of the computer behaving in "closed-loop mode." CO meter reading: Way low. Actually negative numbers, about -1%. The Gunson sucks.

OK, I said. The mixture's lean now, the way it oughtta be for this fuel pressure. So I turned it up, leaving the MAF pot at zero ohms - my thought being to prove it's possible to run a normal fuel pressure, and still have an appropriate mixture. So, up to (only) about 10 psi. Gunson: mid 4% range. AEM: one - two bars rich...with no "closed loop sampling" occuring. Voltmeter on O2 sensor: like 10 mv! (I checked the connections multiple times, changed to a different ground, and also verified the meter read 13.4 volts on the battery. It's an auto-ranging digital meter from Harbor Freight - not exactly a Fluke, but an OK meter.)

So, I turned the fuel pressure down again, my thought being to adjust the FP down until I was about at 1% CO. This occurred at 4 PSI or so again - now, I don't know what the resolution of this Summit Racing, 100 PSI gauge is at this kind of pressure, so give or take. Rechecked the AEM: still reading rich. Voltmeter on sensor is still about 10 mv. Not much change in the way the engine ran when I disconnected the sensor from the plug to the computer, but some.

Well, did I buy a sensor that doesn't work? What do you think now?

Thanks in advance...

Clay

gasmanstl

All brown wires which are hanging disconnected from the removal of an interior/radio/clock, etc are not, indeed, ground wires. In my case, two in a row weren't...so ignore all of the sensor voltage stuff in the last post.

The real truth is, that as the car is sitting now, idling, measuring about 1% CO by the Gunson (if you can believe the durned thing. I have practically no faith in it. I am bidding on another one, with a real, analog needle display.)......

...is that the sensor voltage is about 0.7 volts. This time, it samples periodically again for awhile...until I richen up the mixture to see what kinds of numbers I can get (using increased fuel pressure). With the Gunson purportedly reading in the 6-7% range, it really doesn't change much. When I unjump the idle switch connectors this time, and then jump the WOT side, the CO goes up to the 12% range...and the voltage goes UP to about 0.8. This is the opposite of what's supposed to happen, right?

At this point, I'd welcome a diagnosis of a blown computer...even if it does cost hundreds to fix...just to be done with this. Is it possible that just running rich for a little while could have screwed this sensor? Of course, I had to engage in my electrical idiocy at the BEGINNING of this trial, when the sensor was NEW.

And the AEM...how can it be reading RICH when the O2 sensor is in the .7 - .8v range? Is it broken? Does it need a proprietary sensor or something...and is it compatible with the one I have installed?

I'm soooo confused...

John Speake

Richer mixture = higher O2 sensor voltage reading... so what you find seems OK.

I doubt your LH is faulty.

Disconnect O2 sensor, see if you can get near 1% CO at a reasonable fuel pressure with the MAF pot down at zero ohms.

gasmanstl

Oh. OK. I think, in an earlier post, you had said the opposite:

""""You can of course try with the O2 sensor connected. See if its readings are plausible. e.g. if mixture very rich reading will be 200mV or so. Increasing as the mixture weakens.""""

That's one reason I was so confused... OK, I'll get on it, and advise.

gasmanstl

Well...in a word, no...

Sensor d/c'ed from brain, fuel pressure up to 24 psi: Gunson reads 13% range.

Fuel pressure down to 14-15 psi: Gunson reads 12% range.

Fuel pressure down to 7-8 psi: Gunson reads 10-11% range.

Fuel pressure down to near zero, looks to be a couple of pounds: Gunson in 2.5% range. Connecting the sensor at this point (I didn't think to measure the voltage) results in the CO reading going down very low, Gunson reading a slightly negative number. Then, when I turn the pressure back up to 4-5 psi, the CO reading is immediately way up, like 4-5%.

The injectors can't be *that* much too big, do you think?

Thanks once again for helping to lead me through this morass.

Tampa 928s

Have you taken out and looked at the Inj(s) and made sure you know what you have in there!

gasmanstl

But the previous owner told me he installed 30 lb injectors...and also told me that the car used to run OK with the "previous turbo kit" installed. Truth be known, the car *does* run OK...like a striped assed ape. I wouldn't have started trying to diagnose a problem, except that the AEM was always reading too rich, and I couldn't get it to pass emissions even with a cat, without turning the FP way down.

John Speake

Sorry about the previous incorrect info on O2 sensor output volts versus mixture :-(

The fact that at very low fuel pressure your CO drops after connecting the sensor shows that the O2 loop is doing what is should. The problem is that its range of control is about +/- 20% on injector time.

If you do have 30# injectors, then they flow about 25% more than the stock 24# so you shouldn't have to back off the fuel pressure by much to get the O2 loop within range. Especially if you have also turned the CO pot on the MAF down to minimum.

Injector flow rate will drop by square root of pressure change, so if you are turning 40psi injectors down to 4psi flow rate will drop by factor of 3 or so.

Something doesn't stack up..... like Tampa said.