Re: 156 2.0 TS Big Flat Spot
Now pulls like a train, restoring my faith that the good vibes I got of the engine when I test drove it weren't just a classic case of 'falling in love with the first Alfa you drive' syndrome. But it still jerks on a steady throttle at 80 in top. See the latest iterations below.
It's also booked in for the cambelts and aux belt on Wednesday.
New Alfa lambda sensor went in on Thusday and this restored the idle to almost normal and improved the flat spot so you could nearly drive through it but made no real improvement to the the jerkiness on a steady throttle at 80 in top.
New Alfa MAF went in yesterday and this has made the idle totally steady and as good as it has ever been. It also improved the flat spot but not cured it completely. It now feels like the traction loss traction you get when when accelerating over a pot hole. But most importantly there is no differerence to the jerkiness on a steady throtle.
With the car running like this I took the idle actuator off the throttle body and tied it up. This is with the electrical connector still in place so the system sees no change to anything. The engine cooled off in the time it took to do this and just as you would expect if the idle actuator was working - it took lots of throttle blipping to get the thing warmed up enough to idle properly. Good. That proves the cold start bit of it works. Now for the test drive. Yes you guessed it. No change to anything at all apart from a tendency for the idle to go very low a you come up to a junction and then it' hunts a bit. But again this is good because it suggests that the bit that is supposed to keep the revs up as you dip the clutch is also working. I also squirted through the idle actuator stepper motor with contact cleaner and got a dissapointingly small collection of what looked like iron filings (fragments worn off the brushes ?). The idle returned to normal after I re-fitted the actuator but there was no improvement in the jerkiness on steady blah blah etc.
So, although I find it plausible that another purpose of the idle actuator is to stop the butterfly tending to close completely on a light steady throttle, it seems strange that it is working normally in other situations. It also looks very new compared to anything else in the engine bay suggesting that a previous owner has already tried changing it.
Or looked at another way - I have only proved that it's not the idle actuator doing something it's not supposed thats causing the flat spot. It could still be not doing what it's supposed to do (ie. keep the butterfly open) and hence theres a flat spot
So if it is working some times but not others, this would suggest that it's not being told to work, which must be by the the dreaded ECU or by whatever senses that the manifold pressure has dropped.
Way forward ?
Is this where an ECU reset is required ?
How much is Alfa Diag ? What do I need to use it ? Where do I get the stuff from ? How much does it all cost ?
There are four pressure tappings in the manifold just after the throttle valve ? Looking from the throttle inwards they are;
1. Top left - soft rubber pipe goes to a non-return valve and then to the rocker cover and then into the air intake pipe downstream of the MAF.
2. Top right - a hard black plastic pipe which goes into the bulkhead in front of the driver. Is this going to a pressure transducer for the ECU ?
3. Bottom left - soft rubber pipe goes back into the manifold about 8" further into the manifold.
4. Bottom right - directly into what looks like a non-return valve with an electrical connector on the side. The pipe then runs under the manifold and I can't see where it terminates.
2 and 4 would seem to be prime suspects for investigation. Does anybody what the function of each tapping actually is ?
Can you get carb conversions for Twin Sparks ? Only joking ha, ha !
Last edited by rt05492; 19-04-08 at 13:10.