MAF testing info

[UrS4boy]

In searching for a solution to my hot re-start issue with my UrS4 (after getting no improvement by replacing the G4 crank sensor with a US$205 one), I decided to check out the MAF. As I did so, I checked the pins, cleaned them and added Stabilant 22A contact enhancer.

This is the info that I just added to my G70 MAF post:

For the G70 MAF harness connector the female pins are only in positions 1 through 5; No. 6 the one at the left end of the connector is empty. For the AAN MAF harness connector, the five pins are as follows:

Pin 1 = Brown with red stripe, goes to a ground (16) (on the cylinder head (cam) cover)
Pin 2 = Red with black stripe, goes to pin T26/55 in the ECU
Pin 3 = Green with white stripe, goes to pin T7/55 in the ECU
Pin 4 = Green with purple (violet) stripe, goes to pin T25/55 in the ECU. This is the burn-off signal wire from the Holding Relay
Pin 5 = Black with red stripe, connects to the fuel injector harness

Here is a photo showing those five AAN wires:



Testing the G70 MAF is found in the Bentley in section J24, Volume One, pgs. J24-77 to J24-85. The following is a short-form summary of this info and is not complete:

1. Testing for ground: Disconnect the harness connector from the MAF. Set your DMM to 20 ohms and look for continuity between Pin 1 (Pin 6 is the empty position) and ground on the engine or chassis. Not continuity = you have a ground issue involving that Brown/red wire. Start tracing back, looking for a broken wire. Won't be fun.

2. Testing for connection to the injector circuit: Turn the DMM to 20 VDC. Connect to Pin 5 and ground. Turn on the ignition. You should get some positive DC volts.

3. Checking MAF function: With harness connector removed, switch DMM to 200 ohms. Connect DMM probes to terminals 1 and 2 on the MAF (1 is on the right, 6 is on the left). Record the resistance. Disconnect leads and "Short" the circuit (press probe ends together). Display value must equal value recorded previously.

If the difference is greater than 0.1 Ohm, replace MAF.

Re-connect the MAF sensor harness connector. Peel back the rubber boot on the connector to expose the terminals. Switch the DMM to 20 VDC. Connect the DMM to the back side of Terminals 1 and 3. Turn the ignition switch to "ON" (run) but don't start the engine. Should be 1.2 to 1.5 Volts. Start the engine and let idle. Voltage reading should now be 2.5 volts. Increase the engine speed briefly (manually move the throttle position to more open). Voltage should increase to 3.0 to 5.0 volts (depending on engine speed). All the of the specified voltages must be obtained. If NO = replace MAF.

There is more re: Holding relay/burn-off wire but that shouldn't directly affect running and idling.

The full G70 MAF post: http://forums.quattroworld.com/s4s6/msgs/20442.phtml

[UrS4boy]

My MAF testing results

1. Testing for ground: Disconnect the harness connector from the MAF. Set your DMM to 20 ohms and look for continuity between Pin 1 (Pin 6 is the empty position) and ground on the engine or chassis. My result = 0.5 ohms = continuity = there is a ground for the MAF = good to go so far

2. Testing for connection to the injector circuit: Turn the DMM to 20 VDC. Connect to Pin 5 and ground. Turn on the ignition. You should get some positive DC volts. My result: 11.1 V = good

3. Checking MAF function: With harness connector removed, switch DMM to 200 ohms. Connect DMM probes to terminals 1 and 2 on the MAF (1 is on the right, 6 is on the left). Record the resistance. My result: 0.000 ohms
Disconnect leads and "Short" the circuit (press probe ends together). Display value must equal value recorded previously. My result: 0.000 ohms = same = good

If the difference is greater than 0.1 Ohm, replace MAF. (Nope. All is good so far)

Re-connect the MAF sensor harness connector. Peel back the rubber boot on the connector to expose the terminals. Switch the DMM to 20 VDC. Connect the DMM to the back side of Terminals 1 and 3. Turn the ignition switch to "ON" (run) but don't start the engine. Should be 1.2 to 1.5 Volts. My result: 1.25 V

Start the engine and let idle. Voltage reading should now be 2.5 volts. My result: 2.50 V

Increase the engine speed briefly (manually move the throttle position to more open). Voltage should increase to 3.0 to 5.0 volts (depending on engine speed). My result: 3.05 V

All the of the specified voltages must be obtained. If NO = replace MAF. (Nope, all good)



Bad news is the search for the hot-restart solution continues. Next up: revisiting the TPS because I can get it to restart easier if I crack the throttle, i.e. take it off the idle switch. News at 10.

[prj]

Testing maf is done on hot idle using diagnostics, group 002. Not by measuring obscure voltages.
It's important what the ECU sees, nothing else.

[UrS4boy]

Testing maf is done on hot idle using diagnostics, group 002. Not by measuring obscure voltages. It's important what the ECU sees, nothing else.

Can't disagree but everything that I stated came from the Bentley manual which is based on VAG service bulletins. Obviously they don't know anything.

I don't have VAG-com, etc. just a DMM, so I do what I can do.

[igor136]

Maybe you have injector leak

[UrS4boy]

Maybe you have injector leak

Maybe but they were professionally rebuilt only a few months ago (I think the hot restart issue goes back before then).

[igor136]

I dont think it's the MAF. The car must start without MAF. Blue smoke?

[mike74]

Have you tried priming the fuel pump several times before starting it? Might be a fuel pressure issue?

[igor136]

Have you tried priming the fuel pump several times before starting it? Might be a fuel pressure issue?

[UrS4boy]

Have you tried priming the fuel pump several times before starting it? Might be a fuel pressure issue?

Just did that:

After replacing the fuel pressure regulator 10 days ago (no change) and G4 sensor yesterday (no change) and today checking out the MAF (all good) and the TPS (all good), it was starting to point at a fuel pressure issue, e.g. leaking check valve at the fuel pump.

So I decided to use my switched fuel pump testing power cable, to pressurize the fuel rail prior to starting the car with the key (i.e. by-passing the FP relay regarding power to the FP):



As per the fuel pump test, I removed the FP fuse (20 Amp, No. 17) and connected the clip to the (+) terminal on the cowl (switch off). Then I turned the switch to on and waited for the sound of the fuel pump running. I got that but I also got the sound of a spraying injector, i.e. one that might be periodically stuck open. (Shouldn't be the case, I just had them rebuilt). If that is true, it would make sense why touching the throttle sped up the hot restart: one cylinder (or more) are flooded and the extra air clears that out. No real change in the starting conditions - still needed some open throttle to get the engine to start.

Next step (tomorrow - got to go out and meet my niece and family now) will be to remove the fuel rail and place the injectors into five glass jars and observe, both under static and fuel pump test conditions. I suspect I will find the culprit (yay!?)

Then phone the injector rebuilder and ask for my money back.

Fuel Pump Testing Kit guide:

http://forums.quattroworld.com/s4s6/msgs/260108.phtml

[Morris400]

I had this same issue for the longest time. Change your fuel pressure regualtor. I changed mine and it was perfect after.

[UrS4boy]

I had this same issue for the longest time. Change your fuel pressure regualtor. I changed mine and it was perfect after.

Lucky you.

I guess you missed this:

After replacing the fuel pressure regulator 10 days ago (no change) and G4 sensor yesterday (no change) and today checking out the MAF (all good) and the TPS (all good), it was starting to point at a fuel pressure issue, e.g. leaking check valve at the fuel pump.

[mike74]

Have you had a chance to test the injectors yet? Hopefully thats all thats wrong with it...annoying but relatively easily fix

[UrS4boy]

Have you had a chance to test the injectors yet? Hopefully thats all thats wrong with it...annoying but relatively easily fix

Out late last night. Just up now (8:17 AM here). I will be testing the injectors in an hour or so.

[UrS4boy]

Plan B results.

Out late last night. Just up now (8:17 AM here). I will be testing the injectors in an hour or so.

Plan A was to start the car for 10 secs. Then stop it and remove the fuel rail, placing the ends oft he injectors in 5 glass jars and monitoring them for leakage. Plan B was to be to pressurize the fuel rail again with my fuel pump testing cable to confirm the leakage that I thought I heard. That was yesterday but I had to go out to meet some relatives downtown last evening.

This morning (20 minutes ago) I removed the rail* as part of Plan A. No obvious leakers. Placed injectors in 5 smallish glass jars while still attached to the fuel rail (duh). Used the FP testing harness to pressurize the rail. No sprayers. The sound I heard yesterday (and today) is simply the fuel recirculating through the fuel rail (i.e. into the rail, past the fuel pressure regulator and back to the tank via the return line).

Plan A is in progress while I cut the front and side lawns.

*- to actually lift the fuel rail up with the two bolts holding the rail to the intake manifold, you need to disconnect the vacuum line to the FPR and remove the injector (and G40) harness from the fuel rail. I had forgotten about that. No biggy. Zip ties.