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Sun Oct 19, 2008 3:01 am

Hi,

I have a 1990 vanagon 2.1 waterboxer (well a caravelle GL actually here in Australia) which as a hesitation/flat-spot problem on acceleration...

For example you accelerate and not much happens for about 5 seconds then a surge of power and off she goes - there is definitely plenty of power there - runs fine at highway speeds etc but applying the accelerator regardless of RPM always results in a hesitation before the power is applied.

My first thought was the TPS but i've checked/adjusted that... the AFM and Oxygen sensor & grounds check fine... Both temp sensors have been replaced. Doesn't seem to be any vacuum leaks.

It sounds to me like its definitely fuel related since if I floor the accelerator suddenly from idle it makes a 'pop' indicating its running lean for a second.

Any help would be great, thanks :)

Sun Oct 19, 2008 6:34 am

Try Adjusting your AFM if it's popping, it's too lean, assuming timing is set correctly. Check your current setting, then turn the screw in a few turns(clockwise) and see what happens you can always go back. Have fun.

Sun Oct 19, 2008 8:49 am

I know you adjusted the Throttle Switch, but they also can stick when they get older. Unplug it and see if you still have the flat spot. If it goes away, get a new Throttle Switch. The pop you hear is most likely the Evaporative Emissions valve closing at high vacuum.

Contrary to popular belief adjusting the AFM does little if your O2 system works. The O2 sensor will lean or enrich, as needed, to counter your best efforts at adjustment. You will only have an effect if you go over its ability to adjust the mixture. Besides, if you want to adjust any mixture related issues, you would need a real Gas Analyzer to give you accurate results.

http://www.vanagonauts.com/Adjusting-Wiper-Spring159.htm

http://www.dezend.org/index.php/article/pc/2008-09-03/13031.html

Sun Oct 19, 2008 9:06 am

<< My first thought was the TPS but i've checked/adjusted that>>

Aftter you adjusted the TPS waht is the vacuum reading at the throttle body?

Sounds like either the TPS isn't right, or the charcoal cannister might be saturated and causing the problem.

Sun Oct 19, 2008 10:10 am

Thanks for your suggestions :) I'll check 'em out and report back

Sun Oct 19, 2008 11:01 pm

I triple checked the TPS and its pretty much perfect - even measured it at the ECU plug. I checked the timing again and thats correct. Disconnecting the TPS makes no difference to the flat spot.

I also connected a vacuum gauge to the throttle body by removing the vacuum hose that i think goes to the charcoal canister? (is that what you meant) it reads 0 vacuum at idle and when i rev it jumps linearly up to 20INS/Hg... about 2000rpm its about 15INS/Hg - although I don't quite get what this proves. Having the vacuum gauge connected made no different to the flat spot.

I also tried disconnecting the O2 sensor and that makes no difference to the flat-spot. (I measured the O2 sensor and that appears to be working perfectly and responds fast).

The idle control computer also seems to be working fine.

Also i'm pretty sure its running lean during the flat-spot as sometimes it back-fires. I think i've pretty much ruled everything out except for adjustments on the AFM - been trying to avoid that but I think i'll have to have a go at adjusting it unless you have any further suggestions?

Sun Oct 19, 2008 11:50 pm

Check the fuel pressure from the Fuel Pressure Regulator.

Do the resistance checks on the AFM before you adjust.

Mon Oct 20, 2008 12:05 am

Checked fuel pressure and FPR all within specs/works properly. I also checked the restance on the AFM and it varies correctly etc - i've already moved the vane over to a new piece of carbon. I think i'm down to just adjusting it.

Mon Oct 20, 2008 2:29 pm

I had a similar problem on my 88 2.1. When accelerating from idle to 2000 rpm it would hesitate/ or not respond as it should. When driving down the road, it would also hesitate until above 2000 rpm. I tested everything and found all were within spec. I found the rubber mount on the idle stabilizer valve was broken on the bottom and no longer holding the part securely. This allowed the hose from the idle stabilizer into the intake body(I think thats what it is, its straight down) to come off. :roll: Saftey wired the rubber mount to the bracket and test drove and the problem is gone. Good luck with your van.
Kyle

Tue Oct 21, 2008 9:11 am

Same problem in my 86 Syncro. All new sensors, inspected AFM and moved the wiper to good carbon, checked wiring/harnesses, TPS, Fuel pressure, etc., etc.
Still have the problem, and I am very interested in hearing about any other possible fixes.
Will check the idle stabilizer integrity now...

Tue Oct 21, 2008 9:16 am

Check both the idle stabilization valve as well as the idle stabilization control unit.

See what you come up with.

Wed Oct 22, 2008 6:23 am

Hey again,

Well I changed my O2 sensor for a new one since the old one seemed to be a bit laggy, It has made a difference to overall performance but that flat-spot is still there. When starting off you have to feather the throttle for a while until you rev past the flat-spot.

I tried adjusting the AFM cog wheel in both directions but after lots of stuffing around and checking with a colourtune i ended up putting it back in the same place as it only made things worse and made no difference at all to the flat spot. Should I adjust the screw adjustment on top of the cog as well? – the screw looks like it’s never been touched i.e. has a dob of sealant on it.

I tried disconnecting the O2 sensor while the engine was running and quickly measuring the voltage (before the computer could adjust) - it always read around 0.7 - 0.9V indicating the engine is on the rich side, however the colourtune indicates that its lean when the flat spot occurs - like i said adjusting the AFM cog wheel makes no difference to this.

Is it possible the fuel injectors may be dirty and causing an initial flat spot even though it revs out ok?

Thu Oct 23, 2008 9:10 am

person wrote: Is it possible the fuel injectors may be dirty and causing an initial flat spot even though it revs out ok?
I'm not going to say that new (or serviced) injectors won't change the problem you're describing but I can tell you that it didn't help me with this problem.

I have exactly the problem you're describing and I've dealt with it by more or less flooring it at stoplight.

Like you, I have checked the TPS a million times and have even replaced the O2 and TPS, fuel pump, fuel regulator, fuel filter, spark plugs - god, everything - with no change. I'm going to try another AFM and see if that changes things. I've also had an occasional backfire which can happen within this "flat spot" as you've so aptly put it.

Keep us posted and I will do the same.

Thu Oct 23, 2008 6:06 pm

deprivation wrote:
Keep us posted and I will do the same.

Thanks for the response about the injectors I havn't got any new ones to try so its good to know that it may not be the problem...

Seems a few folks have this issue so if we can crack this one it'd be great - will keep trying and keep everyone posted. :)

Thu Oct 23, 2008 9:42 pm

I just wonder if your problems may be related to a vacuum leak. they can be insidious. One member had similar issues, only to discover a rusted through bracket on the intake plenum. Only when he was taking it apart did he notice the hole.

Perhaps using a shop vac hooked up to the throttle body to either pressurize or provide vacuum. Will allow you to check for leaks.

Fri Oct 24, 2008 3:08 pm

Dogpilot wrote: Perhaps using a shop vac hooked up to the throttle body to either pressurize or provide vacuum. Will allow you to check for leaks.
That's a good idea - so good, I already looked at this! :D

I live two blocks from a very good Vanagon shop here in Austin and they did a smoke/pressure test and much to my amazement/relief/dismay, I had no vacuum leaks.

This "flat spot" issue was one of those things that I thought was just how Vanagons are supposed to operate, since I've never had a Vanagon. It wasn't until a few people posted about this that I realized the "flat spot" ain't right. The occasional backfire is a real eye-opener, too.

Fri Oct 24, 2008 9:24 pm

deprivation wrote: I had no vacuum leaks.

Yeah I checked again for vacuum leaks but can't find any, I don't think it really fits the symptoms either. Have you tried swapping the AFM?

Sat Oct 25, 2008 10:46 pm

kylemason wrote: After hours of reading and using the search till my eyes hurt, I have as of this afternoon gotten rid of the slow acceleration/hesitation/flat spot from idle to =/-2000rpm. Drum roll please..............

I turned the allen screw that adjustes idle mixture [Bently 24.51 Idle speed and Co2 adjusting] out one turn. This is after checking everything from the plastic cam gear(Thanks Terry K) reading the pro manuals (Thanks Dogpilot) and reading every tuning answer fron 10cent (Huge Thanks for your complete discriptions. You have inspired me to perspire to build a better WBX :lol: )

I just tried this - Found the allen screw was turned in all the way! About 6 turns out and the flat spot problem is 95% gone. If you feather the throttle at take-off there's no flat spot but if you push it down suddenly it appears which leads me to believe that perhaps the injectors are leaky since they are very old and never been checked so ill do that next. If its not the injectors then I suspect that adjusting the AFM cog-wheel a notch might fix it. I'll also have to check the mixture with the colourtune again since I don't have a CO2 meter.

Hope this works for your van :)

Sun Oct 26, 2008 6:09 am

<<It wasn't until a few people posted about this that I realized the "flat spot" ain't right. The occasional backfire is a real eye-opener>>

Here's the dead give away--the backfiring.
And the engne is backfiring upon decleration I'll bet.

The throttle switch isn't set up right, and signaling the ECU when to shut down the gas flow at the right time--OR--the trottle plate isn't shutting down all the way.

The TPS also signals the ECU for deceleration at a precise time.
If it isn't right, you'll get an occasional bang.

I haven't seen a report of how many ohms your multi meter is reading at a closed throttle, and WFO.

I see that a color tune is being used, but no one has mentioned using the CO port prior to the converter.
This is where any poor fuel mixture readings should be taken at-not at the cylinder's with the color tune gizmo.

Make sure your throttle shafts are good and tight and have no slop in them side to side, fore and aft.

Make sure the throttle body to plenum seal is good and in tact.

Take a good hard look at the plenum to intake runner hoses--make sure that they are not hard, oil soaked spongey & leaking.

A real quick way to check for intake leaks at the connections is to spritz them with a shot of vitamin -E.
Not a big blast--just a shot at the connections Throttle body, & plenumn connections.

If the RPM's go up--you know where the engine is sucking air.

It sure sounds like something is amis in the intake area of this engine, and I don't think the color tune is going to tell you much in how the ECU is getting the right signal at the right time from the TPS--or if your having intake leak problems as a sniffer at the CO port will tell you right away.

Mon Oct 27, 2008 7:58 am

Terry Kay wrote: And the engne is backfiring upon decleration I'll bet.
Strangely enough, the backfire happens right off the pedal during initial accelleration - very strange. For me it happens rarely but it shouldn't happen at all.

I rebuilt the throttle and it's tight, the TPS is new as is the cam, no vacuum leaks - it's a puzzler. While I don't like the idea of adjusting things willy-nilly, I may look in to this CO adjust, although I thought that was a setting that was only significant at idle.