Boost reduced when car warms up. Any ideas?

[andrewd]

Iv'e had my AAN engined '93 S4 saloon for around 6 weeks now. It seems fairly rapid, but not quite 230bhp rapid.

I've noticed that when it's cold, and for maybe the first 2-3 minutes of driving, it'll happily show 1.7 to 1.9 bar on the dash guage and feel like it's got the full 230 horses*. After that however, boost is pegged back to a displayed 1.4 bar (I know the in-dash guage isn't reknowned for its accuracy!) and the car definitely feels a bit strangled.

I had the error codes checked on the ECU and the particular specialist I used said there may be an air flow meter fault, but that it was difficult to tell in the hot weather?

I'm intending to drop into MRC when I'm next down that way, but just wondered if any of you gurus had any obvious suggestions? Wondering if it might be a duff temperature sensor somewhere?

(* by the way, I would never normally rev the engine until the oil was nicely warmed through. Just done it a few times to check out my theory!)

[porkkis]

There is some fault. The ecu reduces the boost to the WG base boost of 0.4 bars. For a reason.

You should see over 2 bar absolute on overboost.

[andrewd]

Yeah, I'm aware that the ECU reverts to a wastegate controlled boost level of around 1.4 bar absolute when there's a fault. Looks like that's what's happening here. Guess the question is whether there's anything obvious that would cause this to happen as the engine started to warm up?

Found this very informative page about the various sensors used by Motronic, and what their error modes are. Nothing seems to stand out though.

May try and run through some diagnostics this weekend if I have the time ...

[s2dude]

Yeah, I'm aware that the ECU reverts to a wastegate controlled boost level of around 1.4 bar absolute when there's a fault. Looks like that's what's happening here. Guess the question is whether there's anything obvious that would cause this to happen as the engine started to warm up?

Found this very informative page about the various sensors used by Motronic, and what their error modes are. Nothing seems to stand out though.

May try and run through some diagnostics this weekend if I have the time ...

Sounds like it could well be the wastegate frequency valve...
This is certainly where you should start anyway IMO....often develop faults and cheap at only £35 ~

+ easy peasy to fit.....


Part number for an ABY is

034 906 283 K


hth

edit// just seen you have an AAN so different PN probably. (though it would fit)

[andrewd]

Thanks for that. Likely suspect does seem to be the WGFV. Will check the hoses and wiring first and perhaps try and give it a clean (however that is done?!).

Think the S4 part no. is 034 906 283 H ... worth a gamble for £35 if the other things don't work.

[iain1067]

Clean it with carb cleaner or brake cleaner, and then a light squirt of wd40 or similar. You could also have a small boost leak that only happens when the hoses get warm and more elastic, had that on my old s4, replacing the hose on the bottom of the turbo to the crossover pipe sorted that out, along with a new wgfv that i had replaced before the pipe. it's easier to take the headlight out and remove the wgfv that way, 5 screws that's it

[andrewd]

Cheers for the cleaning tips. Off to my local motor factor for some carb cleaner int he morning then ...

Also just found this page which details how to trigger "ECU Output Tests", one of which cycles the WGFV solenoid. Could be a handy test.

Anyone know if these things are substantially more pricey from my friendly local Audi dealer? (As opposed to, say, www.vagparts.com.)

[s2dude]

Cheers for the cleaning tips. Off to my local motor factor for some carb cleaner int he morning then ...

Also just found this page which details how to trigger "ECU Output Tests", one of which cycles the WGFV solenoid. Could be a handy test.

Anyone know if these things are substantially more pricey from my friendly local Audi dealer? (As opposed to, say, www.vagparts.com.)

They are £35 from a main dealer approx.

May be a bit less (say £5) from vag parts but with carriage and a 5 day wait (ish) its easier from the stealer for this small item....

Though NOT for many other bigger or well discounted bits!! Vag parts rulez

[adamw]

hi i had the same problem! when i bought my 92' urs4 i dident have much experiance with this sort of engines so i thought it was just the way thay worked?! i would jump in the car in the morning and set of to work (giveing it beenz) it would read in the boost gauge 1.9-2.2 then when it reached full working tempature it would drop down to 1.2-1.5!? what could be wrong with it?!

[andrewd]

Okay, been out and had a tinker this afternoon ...

Removed the WGFV (cheers for the headlight-out tip Iain) adn gave it a good blast out with carb cleaner. Air passages were all clear, but I didn't have a handy 12v to test it with.

Plugged it all back in and tried going throught he ECU Output tests from the SJM Autotechnik link, but couldn't get it to progress past the injector tests ...? Tried 3-4 times but no further.

Started car up and let it idle a few minutes, then revved it gently and could hear the turbo spooling a little, and a very definite rapid clicking form the WGFV as boost built.

Took it out for a test drive. A quick foray into boost territory gave an indicated 1.7 bar. Let it warm up fully and tried again .. 1.4 bar. Then another try and reached 2.0 bar, then after that permanently at 1.4.

Let it idle after I got back and listened under the bonnet as I revved the engine. Couldn't hear or feel any clicking from the WGFV at all.

Wondering whether the ECU is actually picking up another fault during warm-up and reverting the engine back to WG controlled boost? Dunno if this is a valid Motronic fault mode?

More head scratching ... perhaps still worth fitting a new WGFV just for the hell of it?

[iain1067]

You pulled the codes???? Don't forget with the ecu output tests that you have to open the throttle to fire each injector, it should then go onto to the next test, i don't remember hearing the wgfv when revved at idle on my s4 or s6, if you are bored nthis weekend pull off the hose i mentined before and have a look for cracking around where the thinner pipe joins the main pipe, that's where mine went.

Iain


Iain

[andrewd]

Tried pulling the codes but don't seem to have a check-engine light (maybe just a US model thing?) and I don't have a suitable bulb/wires lying around. Will see if I can sort something out as I'm keen to see if there is anything stored.

I did open the throttle, and it stepped through each injector but didn't seem to move onto the next test. And then reconnecting the jumper for the next phase didn't do anything?!

Will have a closer look at the hose tomorrow. Cheers!

[S4 CRUISER]

hi mate.sorry to hear bout ur funny boost problem.on the s4's u need to check the hoses for intermittent boost problems, ur problem sounds like the michelin hose must have a split on the ribs under the metal rings.change all ur jubilee clips to new 1's on all boost pipes and intake.pm if u need anymore info.all the best gud luck

[porkkis]

Try to unplug the MFTS and go for a test drive. Dunno if the MFTS overheat signal is wired in AAN harness but if yes this could be it. Faulty overheat signal.

[andrewd]

Sounds like a possibility! According to this page the MFTS also drives the coolant temperature gauge though, and that seems to be reading correctly? Dunno if that means anything though. And it does also state:

"When coolant temperature exceeds 119°C (245°F), the boost pressure is reduced."

Hmm.

Still need to do a thorough inspection of all boost pipework. Dirty hands again tomorrow I suspect :-)