'79 Super sputtering after 20 minutes on highway?

[Vwell]

I have a '79 Super convertible that runs great around town or for less than 15 minutes on the freeway. If I take it much farther than that after I get off the freeway the engine hesitates, loses power and frequently backfires. I have dual baby dells on the engine (synced pretty close--I have a sync tool ordered on the way). When fully warmed the engine seems to run fine, unless at highway speeds. Am I looking at fuel problems? Ignition? Other?

Thanks for any help!

[Multi69s]

If a car starts to run bad after a relatively short period of time, The first thing that I do is to run it without a gas cap. A clogged tank vent can mimic your symptoms. Plus this test is easy and free. If this trick doesn't work, the next time it starts to run bad, pull over immediately and kill the engine. Pull off an air cleaner and look in one of the carbs with a flashlight. Work the throttle and see if the accelerator pump is squirting gas.

If it isn't, you are running out of fuel. If it is, then you probably have an electrical issue. The coil or condenser may be heating up and not giving the engine the proper spark.
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[Vwell]

Thx--that will give me some steps to take.

[wcfvw69]

X2

I've had those same symptoms. One time was the coil going bad, the other time was the fuel pump going south.
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[Vwell]

I followed up on some of the steps you all recommended:
* warmed the engine up, then drove 10 highway miles @ 70 mph until the hesitating, sputtering and misfiring began. Pulled into a parking lot, shut off engine and looked into the carbs. Pumped throttle and saw a gas mist in each carb (not regular stream of gas). I haven't checked that before at temperature--is that normal?
* I've been running a meat thermometer in the dipstick tube, it was reading about 190 F.
* Removed the gas cap to check if fuel tank vent might be restricting fuel supply. No noticeable difference when I resumed driving.

Now that the engine was running badly I was unable to reach highway speeds. Driving stop-and-go on secondary roads I observed some other patterns.
--the problem only occurs under load. When out of gear (at stop light) the engine runs fine, responds nicely to throttle blips, etc. Same when coasting in gear down a hill--no problem.
--Once I reached a certain speed the engine could usually maintain it, but if I had to slow down getting back up to speed was tough. And noisy.
--Upon returning home I swapped out an old Bosch coil that was on the car when I bought it last year. (Had replaced it with an oil-filled Pertronix flame-thrower when I added electronic ignition). Wondered if that would make any difference but after about 10 miles the same symptoms reappeared.

So...fuel problem? Ignition problem? Temperature-related?

[wcfvw69]

Running poorly under load would make me suspect the fuel pump.
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[Aussiebug]

Yes, possibly the fuel pump, but they usually work or they don't, and when they don't, they usually start leaking fuel down into the sump so your oil level rises and it smells of gas.

Power jet blocked? There are two similar looking brass jets in the right side of the carb - idle and power jets. Try swapping them. They are different sizes but that wont matter for this test. If the problem then shifts to the idle and low speed, and you get good power at higher speeds, then you've got a blocked power jet. An overnight soak in carb cleaner (a quick spray wont do) and blast out the tiny passages with compressed air or WD40 using the little straw to get real close, usually clears it.

For info, the idle jet in a 34PICT/3 carb is usually 55 and the power jet is usually 65.

Still wrong/bad coil? There are at least two different "standard" bosch coils, which have different internal resistance. With all wires off the coil, check the resistance between the + and - terminals - you need a coil with about 4 ohms for a stock points ignition system, and the points replacement systems work best with a coil having about 2 ohms. The higher resistance standard coil reduces points-burn. Not likely to cause your problem, but checking this would eliminate one possibility.

190f oil temps are just fine - no problem there. Around 230f and higher mean the engine is overheating.

Not sure about the accel pump squirt. It's usually a nice clean squirt, which should go straight down the throat and not splash on anything on the way down (gently bend the delivery tube a little if needed). But even if the float bowl is half full it should still squirt OK, and in any case the accel pump does NOT affect steady speeds - it only works when you floor the throttle, to help the engine spin up smoothly. But it might be worth taking that test one step further and, when the problem happens, clutch in, turn off engine and coast to a stop (so there's no chance of the float bowl refilling), then check the fuel level in the float bowl. It should be around 19mm from the top. If it's WAAAY down then the "mist accel pump" might be possible and then you do have a fuel delivery problem - blocked lines, leaking lines pulling in air, or a bad pump.

You don't have extra spacers under the pump by any chance? - some folks think that this reduces the fuel pressure but it doesn't - fuel pressure is controlled by the return spring in the pump, and shortening the pushrod lift only reduces the max VOLUME which it can pump. At high speeds, that might then show up as reduced fuel into the float bowl. Engine stuttering from that problem would take a little which to show up too, since the pump would still be providing some fuel and the flue level would be slowly reducing, bring the problem on slowly rather than immediately.

Timing? With the vacuum line off and plugged at the carb (open at the distributor), you should get around 30-32 degrees at 3500rpm. Note that I said 3500rpm. Vacuum/centrifugal distributors get there max centrifugal advance at higher revs than the 009, which is normally set at around 3000rpm.

I doubt that this is your problem since the engine would likely be much hotter than 190f oil temp if the timing was way out, but still its worth checking.

A bad plug out of the box? I've had several bad Bosch plugs and switched to NGKs (B5HS in my case) which have been excellent - the bad plugs would only start playing up when the engine was working hard, so it took a while to diagnose.
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[ashman40]

Dirty fuel filter?
Low speed driving is possible but sustained high speed/high rpm driving pushes the fuel delivery system and a dirty fuel filter limits the flow.
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[Government]

Keep it simple. The only thing changing after 20 min. Is your temp. Start there, aussiebug gave a great checklist so make a list and start at top work it down. Fuel vapor lock or boilage, or blockage, sounds most likely with your description. But it only happens after warming up, sounds like heat, or the case of electric pumps check the wires to pump for corrosion or loose fit. Least it keeps running for you, mine said, "we stop here!" And died out. Idle jets should be checked anyhow at this point, and I would have already pulled carbs to be sure they were not to blame, but they are on the top and I work from top down.

[Vwell]

The car is a '79 SB convert with dual Delloto FRD 34Bs. It was orig fuel injected, and has an electric fuel pump in a front wheel well, probably stock I'm assuming. I'll test both coil(s) but the fuel concerns seem more likely, especially since at temperature I'm only getting a gas fog in the throats, not a solid stream of gas in either carb.

I'll check all wires/contacts on the pump. What might be other reasons why such a pump might start to fail when warm?

[Donnie strickland]

If you're running a stock fuel injection pump, your fuel pressure is way too high. You need around 3 PSI with those Dells, and the stock FI pump puts out 30. Check and see what pump you have.
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[Multi69s]

Okay, lets back up a minute, You say that the fuel is fogging at temps. However, the heat of the engine may make it appear that way (this is called stand off). If you have an access to a 22 LR gun, fire off the round and keep the casing. Then either use small needle nose plyers, or solder a wire to the casing and try and catch the fuel that is coming out of the accel pump. If you can fill the casing at least half full, you have plenty of fuel.

So may people right about vapor lock, yet I have never seen it in any VW in the 30 years that I have worked with them. I guess if your fuel line was sitting on a head or the case, it would be possible. I would also try a new condenser. Back in the day, we never had them fail. However, I have bought new ones that were bad right out of the box.

Hang in there.
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[nocartax]

Read all this and others. - as mine had identical symptoms.

The key problem is misfiring *** UNDER LOAD. ***

New coil fitted today £30 - result. Perfect.

The failing old coil could not create a good strong spark across the much denser fuel/air mixture with the throttle held open for long periods - especially uphill.

(To add though, i had an old Ford Escort, and Fiat panda that both had failing electronic distributor packs - which had identical under load symptoms)

Hope it helps others.