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Tue Aug 26, 2003 6:34 am

Hi. 1976 Westy with the fuel injected 2.0L. I'm having problems with the engine bucking on acceleration. Reset valves, timing and checked the plugs and everything seems fine. The car starts and idles great, but when load is put on the engine it starts bucking. I've installed a new cylinder temp sensor about 6 months ago.

From what I can gather from searches on the board the Air Flow Meter (AFM) can cause this problem. How can I check it to see if it needs replacement?

Thanks!

Tue Aug 26, 2003 7:31 am

If it's a hard "bucking" then I would still suspect the ignition system. Set the points at .016" and use Bosch points, cap, and rotor. The Bentley manual details the procedure to check the AFM with an ohmmeter. Your problem also sounds fuel related since the bus idles find, but bucks when you are accelerating. Could be a clogged filter or clogged fuel lines caused by rust in the gas tank. These are common problems, but they don't usually result in a hard "bucking" problem. That again is usually caused by ignition problems. Good luck.

Tue Aug 26, 2003 7:40 am

Thanks. Forgot to mention I've got a Pertronix electronic ignition installed. Seems to be running within specs.

Tue Aug 26, 2003 7:57 am

If you have the Pertronix and the air gap is set correctly then it may be the AFM. Driving problems with the AFM usually start to occur in a narrow range or band while cruising or accelerating. This happens because the AFM becomes worn on the contact board where the needle makes contact. Sometimes this wear is visible when you remove the cover of the AFM. Check the AFM per the Bentley manual or find a friend with a 1975-76 bus with a 6-pin AFM and switch the parts around to see if it helps. I definitely wouldn't buy a new AFM unless I was positive that was the problem. Far too many people have wasted their money doing this only to find out it was something else. You may also consider doing a flow test off the fuel rail between cylinders #3 and #4. The flow test is simple, the cost is free, and will tell you if your fuel lines are clogged. Use at least a 4'-5' long piece of fuel line to get safely away from the engine compartment and keep a fire extinguisher handy. If you have good fuel flow then you usually (but not always) have good pressure also. Please don't waste your money on new parts unless you are postive about the problem. Good luck.

Tue Aug 26, 2003 9:57 am

Cool. I was wondering if I could assess the AFM's wear by removing the top. I'll do a visual inspection and read up on electronic tests in the Bentley. I'm pretty sure fuel flow is good....replaced all fuel lines a few months ago and checked pressure then when I had a temp sensor problem.

If the AFM turns out to be the problem, any suggestions as to where I should buy a new one? I noticed that Bus Depot sells rebuilt for $150 and new ones tend to be $300+. Anyone have experience with a rebuilt AFM?

Thanks for your time.

Sun Sep 07, 2003 9:57 am

Finally got some time to look at the bus. We've been driving it gingerly and it idles well. AFM checked out just fine -- looked pretty new when I cracked the top while doing the tests. Any suggestions about where to look next? Still bogging when a load is put on the engine -- particularly when accelerating in 2nd gear.

Sun Sep 07, 2003 12:18 pm

You seem pretty confident that the fuel supply is good (new pump last year, checked fuel pressure, filter) so what level of vacuum are you getting? Is it normal or low?

Sun Sep 07, 2003 12:38 pm

Yes. I think fuel flow is fine. I need to doublecheck though. Vacuum? To be honest, I don't even know how to check vacuum nor do I have the requisite gauge. I'll read up on it.

I did try looking for leaks in the vacuum hoses today. I closed my hand over the air intake to see if I could hear any hissing from other hoses -- the idle went down a bit but the engine didn't stop like I expected. I suppose it is not an air tight system. Re-checked timing and temp sensor 1.

In checking the throttle microswitch, I was not able to get my test light to turn on. However, when using an ohmmeter, the resistance did decrease when the switch went on. I guess it's working. Possible that the leads are reversed? I don't think this is the problem though as even moderate acceleration makes the engine bog and the microswitch is only activated when the pedal is almost to the floor.

I feel like I'm out of things to check in the FI system!

Sun Sep 07, 2003 12:45 pm

Since it's a hard miss or bucking then it still sounds like an ignition problem. Use a strobe light to see if your distributor is advancing properly and smoothly. You should still have the vacuum advance distributor with FI. Vacuum problems caused by out-of-adjustment valves can cause problems like you describe also because it sends incorrect signals to the distributor. Plug wires, cap, rotor, or distributor could all be suspect.

Sun Sep 07, 2003 1:33 pm

To get a little more information on your ignition system, you can hook a timing light up to the high tension lead from the coil to the distributor and run the engine. Point the light at the side of the engine compartment and watch for awhile to see if the flashing is consistent. Or hook it onto each of the spark plug leads in succession to see if one cylinder (wire or plug) is causing the problem. I sometimes do this while driving around and watch to see if the bucking corresponds to a disruption in the flashing pattern. This test can give you a heavy hint about whether to be checking the fuel or the ignition systems.

Sun Sep 07, 2003 4:47 pm

pogolo wrote: Yes. I think fuel flow is fine. I need to doublecheck though. Vacuum? To be honest, I don't even know how to check vacuum nor do I have the requisite gauge. I'll read up on it.
It's only $20 at pep boys, etc so you might as well get one. If there is any doubt that the engine isn't breathing correctly the gauge will confirm it. When you do find a leak you'll need to gauge again to verify that you fixed it properly.

One thing to remember is that a low vacuum reading is caused by a low idle which could be caused by a leak. Just because the vacuum is low doesn't mean you have a leak but once the engine has warmed up and you can see that the reading is still low you can assume you have a leak if the dwell, timing and valves are set correctly.

Here's what's normal:

Altitude will affect the reading, but in general 17-18" of vacuum at idle is ideal, 15" is the minimum vacuum that you should drive the bus with because it means the lean mixture will burn up the engine. Anything in between those readings is a varying degree of leakage that's either acceptable to you or not.

The tighter the seals, the higher the vacuum up to the point that the engine is breathing normally at the idle rpm. If you scower the net for information on vacuum you'll think the engine is sick because the vacuum of the average old american car was in the low 20s.

So how does vacuum work? When you open the throttle the vacuum will drop to near zero. That's because there is a huge rush of air entering the engine. Once the engine recovers the vacuum will rise back to the idle level and higher as you keep the revs up. When you snap the throttle closed, the vacuum will spike as the engine is temporarily starved for air. You can see from the above description why all the hoses run around the engine: they are controls.

The vacuum gauge was a traditional tool for diagnosis the engine condition. Watching an RPM meter especially a digital one just isn't as useful as watching the gauge:

- small fluctuations means a timing or ignition problem
- large ones mean a valve problem or fuel delivery problem
- low readings, indicate leaks or compression problem
- low readings off idle can indicate the exhaust is restrictive
- sharp drop from normal can mean a leak at a sealing surface like the cylinder heads

Quote: I closed my hand over the air intake to see if I could hear any hissing from other hoses -- the idle went down a bit but the engine didn't stop like I expected.
That will just choke the engine while you do it. There is so much noise from air moving through the throttle body and air plenum that you won't be able to hear a leak. Bad leaks make the engine run so rough it effectively masks the sound.

If you have an EGR system then the noise of the air moving through the pipe to the air plenum will make it even harder to hear. You need the gauge to be able to adjust the mechanical EGR system. While the older vacuum/electric style is activated by the throttle switch you still need to watch the vacuum change to test it.

Someone told me a trick that didn't work for me but I'll pass it along. Use the cardboard tube from a roll of paper towels to help your ear isolate the noise.

In practice I think best method is to use either starting fluid or lighter butane and spray it around the suspected leak area. When you do it, the revs will rise for a second as the fuel enters the intake and you can see the vacuum rise on the gauge. Butane is safer than propane which is safer than starting fluid. Using carb cleaner is also a possibility but I've had poor luck with it.

Sun Sep 07, 2003 5:02 pm

Just to rule it out, I would also change the fuel filter. The gas tanks in buses tend to rust, especially if it sat up for a while with a partial tank of fuel. When the fuel filter is partially clogged, you can usually crank the vehicle and even rev it, but it will die under load when fuel consumption is highest. If the fuel pump was bad it wouldn't run- electric fuel pumps usually work or they don't.

If the MAF turns out to be bad, I would inquire about a used one on Type2.com and TheSamba, but then I'm kinda cheap :wink: [/i]

Mon Sep 08, 2003 6:25 am

Thanks for the replies everyone! I think the ignition system is fine -- fairly new wires, replaced the cap this weekend just in case. Timing is fine, starts and idles great. The problem occurs when load is put on the engine. I do need to check the fuel filter (again).

Back to the vacuum gauge....um....where do you plug it in? Brake hose? Somewhere more obvious?

Mon Sep 08, 2003 10:49 am

Tee in the vacuum gauge at the nipple between the 1/2 cylinders on the air plenum. That hose leads to the pressure regulator and you should tee in rather than leave the hose plugged.

On some earlier FI busses (I can't remember the cut-off) the tee is on the other side of the plenum.

This isn't the best diagram but look where the red line leaves the air plenum.

Mon Sep 08, 2003 8:57 pm

pogolo wrote: The problem occurs when load is put on the engine.

For your continuing diagnostics, a load based miss can be caused by an ignition problem. The minute you tromp on the accelerator, you have introduced a pile of air/fuel into the cylinders, compression pressures skyrocket, and a marginal plug or a marginal connector resister, or a marginal rotor even, can disrupt your sparks at the plugs.
So can a marginal coil. Don't rule out ignition which is the usual cause of load-based missing.
Colin

Tue Sep 09, 2003 6:44 am

Pretty much at a loss. Replaced the fuel filter -- no improvement. Blue Bosch coil is only a few months old, replaced the sorta worn distributor cap 2 days ago, rotor is only a couple months old -- timing checks out. I'll check the voltage on the coil and I need to get a vacuum gauge to check vacuum. Any suggestions on how to check a Pertronix? I guess just check dwell, right? I'm pretty close to bringing it to the shop, but I've got a little fight left in me.

Thanks for everyone's help!

Thu Sep 18, 2003 7:27 am

Still dealing with this problem. Sorry to bring it up yet again. I'm wondering if anyone can tell me how to test my Pertronix electronic ignition to make sure it's working correctly?

Thu Sep 18, 2003 8:46 am

I had a similar problem on a trip to main - WE DID EVERYTHING to try and fix it. In 2nd, 3rd and fourth, the bus would chugg out and had no power - it turned out to be BAD GAS!!!! We replaced the points, etc and the fuel pump and filter - adjusted the AFM, everything - and nothing worked - eventually i limped back to the camp ground and drove around (limping) until almost empty -

I then filled up a mobile station and added dry gas. low and behold - a miracle occured!! We drove back to NY with the engine running GREAT!!!

Thu Sep 18, 2003 9:34 pm

I've had quite a few instances in my shop of this occurring on both fuel injected Busses and Beetles. Every time, it was the Petronix conversion. For some reason, the ignition conversion and the injection system just quit getting along- even if they've been fine for years! I know that points and condenser are stone- age, but keep in mind that these cars were DESIGNED to run with points. I agree with everyone here who says it's an ignition problem. In a pinch, your Bus will run OK with the good old Bosch 009 distributor. If you use the 009, just make sure your base timing is NOT advanced even an iota beyond spec. AND- change the spark plugs if they're old. Another nemisis of the VW is the platinum spark plug. Use the PLAIN Bosch or NGK. I've seen Platinums (INCLUDING the new Bosch +4s) CAUSE more runability problems than you would believe! If you have Platinums in your Bus, START THERE FIRST. If not that, then I'll bet getting rid of the Petronix and maybe even going with a 009 will do it.
One last thought- If your Bus has an Oxygen sensor, try disconnecting it, taping off the wire, and driving it like that. If it perks up, replace the )2 sensor. Best of luck to you.

Thu Sep 18, 2003 10:05 pm

pogolo wrote: Still dealing with this problem. Sorry to bring it up yet again. I'm wondering if anyone can tell me how to test my Pertronix electronic ignition to make sure it's working correctly.
The tests are the same for points: pull the wire to check the color of the spark and use a dwell meter which will read about 54 degrees. The only other way I know is to replace the points and condenser and go for a drive.

tramandbixrule@yahoo.com wrote: I know that points and condenser are stone- age, but keep in mind that these cars were DESIGNED to run with points.
The same argument could be made for bias ply tires.

In the bus ignition system, the coil and battery and spark plugs are doing all the work. The points are just the trigger. Points degrade long before the service interval (with the exception of those with blessed points the rest of us can't seem to purchase) and although that's by design, it's also inconvenient.

Electronic ignition and VWs has a bad reputation almost entirely from the accufire and magfire. It was the cheapest knockoff of a reputable version that was made in Taiwan and had a 90% failture rate according to the vendors who sold them.

All I can say about the Pertronix is that I'm happy and it has a 30 month warranty.

Pogolo,

I just reread all the messages again. How about checking the total advance? Maybe the vacuum can on your distributor went kaput.