High rev problem

I have an '85 Golf Ex-Cup Car. I recently rebuild the motor, and everything runs smoothly. Or at least it does below 5k RPM. Over 5k, it misfires horribly and has no pull at all. Adjusting the timing and air sensor plate make no difference, so I started looking deeper.

I finally got down to the Differential Pressure Regulator current, and it was 0 mA. I checked the voltage at the connector, it was 0V. The wires are continuous back to the computer, so it looks like the computer is not sending a signal.

Does this mean my computer is dead?

Would a dead DPR cause the engine to lean out at high RPM?

Is there anything else I should check?

Thanks,
Brian

Brian
Is your car a GTI or a Golf?

Looking at the wire diagram do you have voltage at pin 1 of the CIS-E control unit and is there grounds at pins 2,15,22?

What is your O2 voltage at idle and at 5K?

If it is a GTI it will have a knock sensor control unit, do you have voltage at pin 5 and voltage at the middle pin of the throttle switch plug and middle pin of the idle motor and grounds at pins 3,11 of the knock box

The car is a mutt. The Cup cars were built from parts on-hand, so it's always an adventure when I work on it. As an example, the engine block is stamped with the code for the diesel engine. Beyond that, it does not have a full throttle switch, so I think it was orginally built up as a Golf and not as a GTI.

I will check those pins when I work on the car this evening.

There is no OXS.

I did some more research last night after I posted and have a few more ideas. Is it possible that without an OXS input, the computer is going into limp-home mode and not sending a signal to the diff. pressure regulator? If so, would that cause the engine to lean out as the revs increase?

Have you checked the DPR current with the car running. If it is zero, that is not right. If someone was running a "fuel enrichment" module it is possible to overload the output driver and poof. If you are getting no current it is as good as just disconnecting the DPR. I am surprised it even wants to rev up with no load like that. Check that current.

Many of us run with no O2, it will run open loop, but it will not be limp home. You can add a manual switch for the full throttle and you might possibly just need to tweak the fuel delivery.

(it sounds like it is running lean)

The DPR current is 0 mA with the car running. I was afraid that the computer might have lost that circuit. Looks like I'll be getting another one.

Is the computer for the GTI and non-GTI the same? Will the non-GTI computer accept a full throttle switch input?

Thanks,
Brian

I don't have the bently with me, but I also didn't know that other CIS-E cars had no full throttle, so I know a red tag box or GTI box have all the same inputs. You might want to look for one of them. Full throttle switch will set open loop and increase current to the DPR.

The computer uses O2 sensor voltage to adjust fuel mixture. With out an O2 sensor ther is no reason for a DPR. Are you sure of the fuel system you have is a street version or some hybred thing for the cup cars. Just to be total clear here, you know the DPR is the grey box on the side of the fuel dist and not the alum. box bolted to the front of the block at the fuel pump block off next to the oil filter housing?

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Originally posted by tschwenke:
so I know a red tag box or GTI box have all the same inputs. You might want to look for one of them.

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Don't go by the color of the tag. I just sorted a few milk crates 2 weeks ago so still relatively fresh in my mind, but no guarantees...

Some CIS-lambda boxes have red tags with a 263 part number!! CIS-E boxes have red or green tags with a 264 part number (various letters afterwards). Don't remember the color of the base CIS-E boxes, but I think 8V GTIs were green and 16Vs were red.

The key is really the "264" for CIS-E, and the letter that follows specifies the original fuel map (and possibly full-trottle enablement and/or percentage, decel. fuel cut-out, etc.), FWIW.

Yes the system does use the O2 sensor, but not at full throttle and if you ever want the system to deliver enough fuel to get to a better AFR you don't want the 02 sensor. It is shooting for a AFR that is much higher than what you want. You must tune the fuel system after removal. Lots of us also run 100 octane unleaded and the 02's don't seem to like it. This is not out of necessity, but more out of cheap knock protection, one can run fine on 93 but it is not recommended by me for sure.

And, the DPR still will respond to current as it was designed and even without the O2. Example: the system will still send different values dependant on coolant temp/full throttle switch. Unplug the DPR with the O2 gone and the car will not run, plug back in just the DPR and it will run fine, most likely putting out around 8-10mA. Hit full throttle and you will see the number increase.

Thanks for clearing that up on the tags, I was only refering to the color of the base CIS-E colors.

The new computer is in the car. I am getting a DPR current now. It runs just as bad as ever.

Here are the latest results from the tests on pages 25.36-37 in the Bentley.

Open temp sensor, ignition on: should be 80-110 mA, is 59 mA
Lift airflow sensor plate: should jump over 110 mA, jumps to ~98 mA
Shorted temp sensor, open OXS: should be 9-11 mA, is 9.9 mA
Shorted temp sensor, grounded OXS: should increase to 19-22 mA, is 9.9 mA
Car running, temp sensor connected, grounded OCS: 1.1 mA

The wiring harness passes all test on pages 25.52-53. I have a fuel pressure gauge on the way so I'll be able to test that soon.

Something BSI (Stu) has told me before and I have used more than once.

Pull all your injectors, short fuel pump on, turn the 3mm until they drip, back the 3mm off 1/2 a turn. Basic adjusment done.

Remember where the current position is of the 3mm. A couple of measurements are concerning, but they shouldn't be causing it to not run. I would check the resistance of the coolant temp sensor as seen by the cpu. Use the bently chart, you should see it at the Ohms for your current ambient temp (as long as the car has been sitting there), unplug and check that the CPU sees it open. Check all grounds going to the CPU. I would also for giggles just unplug the O2, unplug the cold start injector, and unplug the idle air stabilizer. Either screw the idle adjust screw in all the way and back off 2 turns or use some crushed cardboard to "open" the throttle a tiny bit. It should idle (around 1500) and drive fine with all those "important" items unpluged.

When you say bad, just no pull above 5k or won't even idle.

You might be strapping the fuel pressure gage to the window to see while driving. Also, you can watch the voltage going to the fuel pump under WOT to see that the pump is getting good consistent power. Big change in Voltage = big change in fuel pressure.

I've also replaced the coolant sensor on my Rabbit and gotten more consistent start up and idle.

Since it's a race car, I also removed the idle air auxiliary valve and blocked off the hole - less to leak/fail.

PJ

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83 Rabbit GTI - future ITB

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<font face=\"Verdana, Arial\" size=\"2\">Since it's a race car, I also removed the idle air auxiliary valve and blocked off the hole - less to leak/fail.</font>

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PJ, would that be the idle adjustment on the throttle body? I had one of those vibrate out last year. Didn't notice it until I came off track, and was wondering why it was idling @ 3,000 RPM. Noe I carry a spare or 2 taped to the upper lid of the tool box, right next to a couple of spare cam gear keys.



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Tim Linerud
San Francisco Region SCCA
#95 GP Wabbit (Bent)
http://linerud.myvnc.com/racing/index.html