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Hesitation/ Bucking on load

I recently started having a hesitation problem on full acceleration! The timing is perfect, the plugs are great, the distributor is perfect, the plug wires are new, the ignition coil is great, the fuel injectors were poptested and all passed, the WUR is new, fuel distributor is rebuilt! Upon starting the car it feels like the cold start valve helps to relieve the hesitation. I can rev up the car and there is no hesitation at all until the thermal time switch shuts off the cold start valve then that is where the hesitation is noticeable! The idle is rock solid when the engine is cold or hot! The rear fuel pump is a brand new unit! But the one in front is a rebuild that was put in 2.5 years ago! The rear fuel pump works perfect. Oddly the front fuel pump upon starting the car does not seem to activate the red relay in the fuse panel as it used to! I seem to only feel the hesitation/ bucking when the car is being driven around 2000rpm to 3000rpm and then it clears up! I believe that if this was an air leak issue then I'd see a hunting idle! And if the CO% was out of place then the idle would be osculating! But in my case the idle is rock solid!

Also if one of the cylinders was leaking air, could the idle still be rock solid, or would it affect the idle? Would it only affect the drivability on load?

Old 12-22-2006, 11:02 PM
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A little more info is needed:

What model year, carbs, CIS....

The last hesitation problem I had was a bad ignition coil.

When we have some more vehicle info more will chime in...
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Old 12-23-2006, 02:07 AM
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When my '74 with CIS did this (hesitation/bucking under load), it was a combination of fuel distributor problem and ignition problem solved by converting to Webers and replacing the Pertronix ignition wih old-fashioned points...
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Old 12-23-2006, 03:34 AM
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You could check the fuel pressure and flow volume.
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Old 12-23-2006, 07:19 AM
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Richen it up a bit to see if it goes away....
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Old 12-23-2006, 07:42 AM
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It's a 1979 930 Turbo.
Old 12-23-2006, 09:58 AM
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Check the obvious first. Fuel filter. Does the hesitation get worse as the rpms increase? If so this is definitely a fuel delivery problem.
Old 12-23-2006, 05:01 PM
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Filter.....
Old 12-23-2006, 05:04 PM
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Your Cold Start Valve should only be operating for a few seconds while the engine is starting.

I ditto the mixture setting suggestion. You may be just slightly lean. Even though the WUR is new, it wouln't hurt to check your control and fuel pressures.
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Old 12-23-2006, 05:46 PM
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Yesterday I got my front fuel pump! So I did the installation today and found out that the new fuel pump was leaking from the seam! It's a new Pierburg fuel pump. Not a rebuild but a new one! I figure that some idiot dropped it on the floor. So come next week on Tuesday I'll return it. I hate it. It feels as if someone doesn't want me driving this car! I did change the fuel filter just last week too, so that doesn't seem to be the problem. I heard in a previous post that someone reported saying that it was possible to run a 930 with a single fuel pump. He did so, but it caused the car to run rich! He just ran the front fuel pump and disconnected the rear one! I did a test today on the front fuel pump. While I started the car I had someone put their finger on the red relay in the front fuse panel. The relay buzzed for a few seconds then shut off as the car began to run. Then the next 10 seconds the car ran perfect! I could rev it up with ease! I assume that the cold start valve had something to do with it. As it shut off I revved the engine a few more times until it started misfiring, then the fuel pump relay started buzzing again! That pesky front fuel pump! I don't trust it!
Old 12-24-2006, 01:07 AM
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I do not own and I have never tinkered with a turbo Porsche...

That being said, I agree replace the defective fuel pump in the front.

I would also suggest replacing the fuel pump relay(I think the turbo has 2 relays) if you don't know when they were replaced.

Have the A/F mixture adjusted by someone with a sniffer to get it right....this save alot of problems.

The buzzing of the fuel pump relay is concerning. When my SC had a hesitation problem the fuel pump relay would buzz/click because the rev limiter relay (behind the fuel gauge) was shorting thus acting like the vehicle was at redline. The relay shuts the fuel pump down at redline.

I am not sure if this is applicable to the turbo. I mention it because if the front fuel pump replacement doesn't solve the problem it may say you a lot of time fixing this annoying problem.

Good luck and Keep us posted.
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Old 12-24-2006, 02:39 AM
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So I replaced the fuel pump and found the engine to be running smoother, with less stumble. I realize that it's the injectors! I'll get on it and bench test them all again to be certain! They should be opening at 2.1 + 1.1 bar
Old 12-29-2006, 12:24 AM
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like I said, it is a fuel delivery problem, so you would start at the source and work your way forward, pump, filter, ect. fuel mixtures, (unless they are way off) will typically only effect performance at idle. I would change the filter as its very job is to become clogged with foreign matter, and in turn become restrictive. The fact that the new fuel pump made a difference suggests a volume/pressure issue. injectors may also be the culprit, but they the final component in the system. Good luck
Old 12-29-2006, 10:09 AM
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I recently had a similar problem. And after spending days doing stuff like checking injector spray pattern and volume, and some other complex stuff, I found the problem. It was a distributor-end spark plug wire connector that was "open." The distributor-end connectors are supposed to have a 1000-ohm resistance feature, but one of mine was "open." Infinite resitance. The spark plug end connectors have, I think a resistance of 3000 ohms. Check them. Total resistance of a complete spark plug wire including the connectors at each end should be about 4000 ohms.
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Old 12-29-2006, 10:30 AM
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relays should not buzz

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Old 12-29-2006, 04:42 PM
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