Deceleration Valve problem

[Marcas]

I am having a fast idle problem which I think relates to the deceleration valve on my 1978 fuel injected superbeetle. When I do the test in the Bentley manual and squeeze the vertical hose that attaches to the air intake manifold beside the auxiliary air regulator the revs drop. However if I let the hose go the revs do not increase. Also if I turn off the ignition and restart the car the revs are at their normal level and no air appears to be leaking through the valve.
I have the original deceleration valve and one I bought as a spare ten years ago and both have the same symptoms. I have tried adjusting the older one as per instructions I found on the web written by Ray Greenwood, but it has made little difference. Before trying to adjust the newer deceleration valve, I thought I would ask if anyone could offer an alternative explanation to two problematic valves.
Just in case anyone suspects a problem with the plumbing, the narrow vacuum pipe goes to tee with one pipe going to the fuel pressure regulator and the other going to the lower vacuum connection on the air intake manifold. The other pipe goes to the connection between on the rubber boot linking the air intake manifold and intake air sensor

[Piledriver]

I'm going to need some good pictures to jog my memory, it was a pretty simple system, but that's ~all I remember about it.

Have the link for Rays walk through? might help resuscitate some gray matter.

[Marcas]

Thanks for offering to help. I have attached some pictures. I haven't changed much in the last 10 years. It is pretty much stock except for a svda distributor from aircooled.net with pertronix and the EGR system has been blocked off. The car is running well apart from this problem and I have sprayed carb cleaner around anywhere there might be leaks to check. Here is a link to the instructions from Ray Greenwood. http://www.shoptalkforums.com/viewtopic.php?p=714658

[Piledriver]

OK, it simply acts as an adjustable vacuum break.

What happens when you do the test described?

Basically it should send vacuum through to the throttle kicker on decel, and do nothing at idle vacuum levels.

You should adjust it so the hose does not suck at idle, and does if you rev it up to ~2700 RPM with no load.
(or slam the throttle shut at higher RPM)

You can neuter it by removing the line, but you may get decel popping//backfires as I guess LJet lacks a decel fuel cut.


Just because you have two doing the same does not mean they are not both broken.
Should be very easy to test though.

[RHough]

You can neuter it by removing the line, but you may get decel popping//backfires as I guess LJet lacks a decel fuel cut.

Some LJet applications have fuel shut. ECU reads idle contacts in throttle switch and RPM ... High RPM + Closed Throttle = Fuel shut.

The air bypass on the systems is to limit manifold vacuum under decel. High manifold vacuum under closed throttle pulls blow-by from the crankcase past the rings and adds to HC emissions. The combination of fuel shut and the air by-pass reduces CO and HC.

IIRC some LJet systems have a solenoid valve to kill the vacuum to the decel valve below some programmed RPM to prevent the idle from hanging up.

Since you have two valves doing the same thing I'd check basics like throttle switch adjustment before going farther.

Cheers,
R

[raygreenwood]

The air bypass on the systems is to limit manifold vacuum under decel. High manifold vacuum under closed throttle pulls blow-by from the crankcase past the rings and adds to HC emissions. The combination of fuel shut and the air by-pass reduces CO and HC.

IIRC some LJet systems have a solenoid valve to kill the vacuum to the decel valve below some programmed RPM to prevent the idle from hanging up.

Cheers,
R

Sorry I did not see this earlier.

Mmmmmm...yes and no. Th decel valve in both L-jet and D-jet can be problematic almost as much for what its SUPPOSED to be doing as compared to what its ACTUALLY doing.

As for the EGR theory....which was never there in the beginning and D-jet had decel valves long before crankcase vacuum was even an issue....and almost all L-jet has a differential pressure bypass diaphragm that limits any issues with crankcase emissions on overrun...but still...OK...sure.

The operative theory is that its supposed to limit vacuum....which is technically does....but giving that as a reason makes no sense as the system in general does not care if vacuum is high really.

What it does is to keep vacuum at high rpms from peaking high enough to drop fuel pressure at the regulator. Remember...that the regulator will drop fuel pressure even
at throttle closed and idle rpm. If the throttle is closed and rpm is high, vacuum can be even higher (not always...but it can be). The decel valve is supposed to bleed enough air past it to keep fuel pressure from dropping all the way down to idle level.

NOTE: Those who still have the belief that the rising rate pressure regulator is designed to match fuel pressure to vacuum situations in the manifold....did not do well in physics. With open area of .015" square or less with a constant high volume fuel feed....the injector flow would not even notice the effects of manifold vacuum leveraged across area that small. In a turbo...possibly....but no one has actually definitively proven that yet. The rising/falling fuel pressure regulator is because PRESSURE X TIME= dosage. Its a primary source of momentary enrichment....sorry for the rant.

Why is this a problem? Because on high speed overrun...like decel from 65 mph in 3rd or 4th on an exit ramp ...will mean that you have excessively lean mixture and are running max centrifugal advance....not a great idea.
Of course....if your L-jet has fuel shut-off system...then the decel valve is only useful in part throttle decel situations as having the fuel shut off gets rid of the problem. No mixture is better than lean mixture at max advance. No combustion is better than detonation I have never been 100% clear as to whether fuel shut off simply shuts off enrichment or shuts off ALL fuel on every system though.

Early on, mostly the automatic transmission cars had the pressure switch and solenoid to trigger decel valve operation or lock it out at certain times. This was a good idea because at many times...especially with an automatic....you could be at technically very low throttle setting at high rpm....like going through a sweeping curve at speed....and a manual vacuum decel valve would trigger a lean situation when you are technically, actually...under load.

Also the key to proper operation of the decel valve with L-jet...is to make sure its plumbed correctly. It must dump in right near the vacuum connection for the fuel pressure regulator in the plenum.....or better yet...T it into the vacuum line for the regulator as its been done on some other L-jet and Digijet/Digifant cars.

The other advertised use for decel valve in primarily d-jet...and somewhat L-jet...was to lean out the downstream mixture (read that as in the exhaust manifold and muffler system).

Typically in systems without fuel shut off.....and in situations when spirited driving is being done...on and off the pedal quickly....or in high rpm transitional driving (the sweeping turn again at part load)......advance may not be maximum yet...or you are fighting the vacuum retard unit if you have one and the throttle plate is barely off its stop......you may have poor vacuum at the FPR...and moderate enrichment at the air box flap....and this leads to excessively rich mixture in the exhaust. Totally drop the throttle and this mixture will combust in the muffler giving popping. The decel valve aids this problem by bypassing extra air to lean the mixture.

And....a lot of that problem is probably more in the exhaust design as well.

This did not really answer the question, but all book explanations for what the decel valve does aside....if you know what the decel actually does and what it reacts to ...and what reacts to its reaction.....its easier to troubleshoot.

I have found that if a properly working decel valve cannot be properly adjusted to keep exhaust popping from happening or to allow smooth throttle transitions....then generally the fuel system it is attached to is probably improperly adjusted.....or the vacuum characteristics have changed enough (new cam? much larger heads and valves? larger manifolds? totally different rpm ranges than stock?)...that the existing valves volume of diaphragm and sizes of orifice are no longer properly matched. Just some thoughts....Ray

[Marcas]

Thanks to everyone who took the time to offer a suggestion. I live in Belfast but have been in France with the car for the last 4 weeks and haven't had access to the internet. Working with the two decel valves that I had I got one of them to behave as it should according to Ray's instructions. I haven't had any popping issues in Ireland before or since, although I haven't yet had time to put up that many miles, but the problem continued in France. At relatively slow speeds when I took my foot of the accelerator on a flat road or when going downhill I got quite a bit of popping which went away if I ran the car in neutral.
On the issue of modifications I rebuilt the engine with standard parts about ten years ago and replaced the heads with new vw fuel injection heads and valves about 4 years ago. The only significant modifications are an svda distributor from aircooled.net and a pertronix ignitor module.

[RHough]

The air bypass on the systems is to limit manifold vacuum under decel. High manifold vacuum under closed throttle pulls blow-by from the crankcase past the rings and adds to HC emissions. The combination of fuel shut and the air by-pass reduces CO and HC.

IIRC some LJet systems have a solenoid valve to kill the vacuum to the decel valve below some programmed RPM to prevent the idle from hanging up.

Cheers,
R

The operative theory is that its supposed to limit vacuum....which is technically does....but giving that as a reason makes no sense as the system in general does not care if vacuum is high really.

The issue of high manifold vacuum pulling HC past the rings and pumping it out the exhaust has been a known problem for emission control since before EFI was common. The EFI system does not care if overrun vacuum is high, those are fuel shut conditions. It is very hard to get reliable combustion when the engine RPM is high and the manifold vacuum is high as is the case when the throttle is closed and the engine is at cruise RPM. Anything in the cylinders gets pumped unburned out the exhaust. Unburned HC are a no no to pass emission tests.

What it does is to keep vacuum at high rpms from peaking high enough to drop fuel pressure at the regulator. Remember...that the regulator will drop fuel pressure even at throttle closed and idle rpm. If the throttle is closed and rpm is high, vacuum can be even higher (not always...but it can be). The decel valve is supposed to bleed enough air past it to keep fuel pressure from dropping all the way down to idle level.

NOTE: Those who still have the belief that the rising rate pressure regulator is designed to match fuel pressure to vacuum situations in the manifold....did not do well in physics. With open area of .015" square or less with a constant high volume fuel feed....the injector flow would not even notice the effects of manifold vacuum leveraged across area that small. In a turbo...possibly....but no one has actually definitively proven that yet. The rising/falling fuel pressure regulator is because PRESSURE X TIME= dosage. Its a primary source of momentary enrichment....sorry for the rant.

You can't have it both ways. Either the injector does flow differently based on pressure differential as would be required to support the "primary source of momentary enrichment" you state or "the injector flow would not even notice the effects of vacuum leveraged across area that small". Which is it?

When I was certified on L-Jetronic back in the dark ages the function of the fuel pressure regulator was to maintain a constant pressure differential at the injector. When manifold pressure is low (high vacuum), fuel pressure measured at the fuel log is low. When manifold pressure is high (low vacuum) the pressure at the fuel log is also high. The pressure differential at the injector is the same throughout the operating range. The same regulator increases the fuel log pressure to maintain the differential when a turbo engine is on boost. Mixture is controlled with injector pulse duration, not fuel pressure in a L-Jetronic system. The fact that when the throttle is opened the fuel pressure reading at the log goes up leads many people to assume this causes a rich mixture. The reality is the pressure at the injector tip (at manifold pressure) remains constant so there is a direct relationship between injector open time and fuel "dosage". Other systems may use variable fuel pressure for enrichment but not L-Jetronic. The basis of the L-Jetronic system is constant pressure relative to manifold pressure. Metering is from the ECU based on mass air flow (Hence L (Luff) Jetronic). If you monitor Fuel Pressure, Manifold Pressure (Vacuum), and Pulse Duration on a L-Jetronic system you will find that acceleration enrichment is due to pulse duration (injector open time) and the relative pressure at the injector remains constant.

Why is this a problem? Because on high speed overrun...like decel from 65 mph in 3rd or 4th on an exit ramp ...will mean that you have excessively lean mixture and are running max centrifugal advance....not a great idea.

Why not? There is no combustion to speak of under those conditions. That is why you can use a fuel shut off system with no impact on drivability. At best you are feeding the engine an idle volume of mixture, there is no load so impossible to hurt anything except the high HC pumping out the exhaust.

Of course....if your L-jet has fuel shut-off system...then the decel valve is only useful in part throttle decel situations as having the fuel shut off gets rid of the problem. No mixture is better than lean mixture at max advance. No combustion is better than detonation I have never been 100% clear as to whether fuel shut off simply shuts off enrichment or shuts off ALL fuel on every system though.

On L-Jetronic systems the fuel shut system is a 0 pulse duration in the furl shut range. There is no possibility of detonation on decel. The closed throttle prevents the cylinders from filling, so compression pressures are very low. In many cases too low to support any combustion at all (hence fuel shut or throttle openers to keep HC low under those conditions). Detonation requires a heat source to ignite the mixture. Under decel conditions there is not enough mixture and there is no heat to ignite if mixture was present. In the early days of emission system development we saw devices to break the vacuum under decel to keep the combustion process going. This allowed the engines to burn the crankcase vapors sucked into the combustion chamber during decel and also reduced the volume of those vapors since they served to limit the peak vacuum inside the chamber. How many CA emission throttle openers have been pulled off VW engines? They were the mechanical solution for a carburetor that the decel valve / fuel shut system on a L-Jetronic is.

*Disclaimer
I learned the L-Jetronic system in 1977 when I went to work for Datsun. I was a master technician for many years and was part of the technical information exchange committee that provided feedback to Datsun/Nissan engineers. The same operating principles applied to L-Jetronic systems into the late 1990's when I retired. If there are new systems based on L-Jetronic that operate differently my statements do not apply.

Cheers,
Randy

[Piledriver]

Any exhaust leaks? Known aggravating issue.

[Marcas]

Thanks for the comments/suggestions. With respect to the exhaust I had a quick look as a friend had suggested that possibility but I would need to have a more thorough examination. I am fairly certain that the exhaust system is OK but since it is a pattern part, not original VW, it may not be the same specification. There is always that possibility of a bad gasket.