For the last few months my bus has had high idle revs when “cold”. It’s most noticeable when coming up to some traffic lights and stopping. The revs drop, but then they shoot up quite high. Once warmed up, after about 5-10 minutes, the problem stops. Engine is a 2000cc Type 4 FI. Any ideas?
Hello, Does it alter with the brakes? If so have a look at the check valve at the engine end of the servo pipe. Or try removing the servo pipe and plugging it to see if it behaves the same. If it is not related to braking, maybe check resistance on the TS2. Also the AAR could be staying open slightly too long and of course there are vacuum leaks to check for.
Try the vac pipe from the fuel pressure reg to the decal valve! Mine made quite a difference after I changed this one.
I remember reading about the decel valve. The Bentley book explains how to test it, and according to that it's working fine..... But maybe the pipe itself is the issue.
Possibly, maybe... I don't often use my handbrake, but on the few occasions I have, you might be right in that the revs return to normal. Gives me something to look at, thanks.
Is this the check valve, just to the left of "A"? https://www.vwheritage.com/191611933-non-return-valve-for-brake-servo-vw-spare
Hi Andy it sounds like a temp sensor or cold start issue to me, not withstanding what the others guys have said ref the vac pipes and decel valve. Check all the pipe connections and test the mechanical stuff as suggested. Maybe then work through the Bentley procedures for testing the FI system. It looks a bit daunting initially but once you get your head around testing resistance and continuity its quite simple. Often a poor contact or a break in a wire can cause odd symptoms, that unless you work through systematically, will drive you nuts! Ask me how I know....
I replaced all of the small braided hoses with new silicon hoses. This didn't improve things. But maybe there is something wrong with the plastic pipe.
I was hoping it would be a pipe or hose before I had to start probing around on the FI system! Maybe that'll give me something to do one weekend. Cheers.
To check the check valve just take it out and blow through it, then turn it around and do the same. It should not let air back through (towards the servo) With regards to the decel hose, if you have replaced it recently, it could be the decel valve itself starting to fail. To test that just disconnect it and plug the hoses and take it for a spin and see if there is any change. Have you noticed any popping/backfiring when letting off the gas?
That could be a sign of the decel valve failing. When working it makes the mixture leaner when you decelerate so there isn't any unused fuel passing through your exhaust. If it isn't working, the mixture will remain rich and could be detonating as it exits. As Busmonkey said they are all easy things to check, you just have to be systematic in your approach before getting tempted to start spending money replacing stuff.
The way the Bentley manual says to test the decel valve is to compress the large hose when engine running. The revs shouldn’t change, which they didn’t. More testing required I think!
The vac pipe S that connects at T and to the fuel pressure regulator the other end (in the bottom corner at back on right and just in front of fire shield. The decal valve is the one in the middle top between pipe connectors D and T and has two larger vac pipes on left ref C and E
