Pelican Parts Forums

Pelican Parts Forums (
-   Porsche 914 & 914-6 Technical Forum (
-   -   Bucking/Surging Causes? (

3D914 09-22-2003 06:49 PM

Bucking/Surging Causes?
Geoff mentioned in a post some time back three of the causes for "bucking" as follows:

1) TPS worn contacts - new TPS, set and ready to go.
2) Worn distributor -
  • wobble in center shaft - Mine was rebuilt recently
  • sticking adv plate - plate appears to move freely (don't use adv or ret)
  • bad points - points are new and dwell is constant
3) Bad FI points - need to check these.

Are there others?

NOTE: My bucking occurs under load. It will idle fine and rev in neutral with no problem. Once I get near 2500-3000rpm and up in 3rd or higher it starts bucking noticably.


RustyWa 09-22-2003 08:23 PM

Sounds just like what mine is doing. Right around 2500 RPMs it's feels like a cylinder drops off. To weird. I can be pulling in fourth gear and it's fine, but when I hit 2500, that's it.

I've got a new TPS, but that didn't help. I'm trying to get a rebuilt distributor now. My old one is pretty sloppy in the shaft. Plus, the rebuilt dizzy should have refurbished (or new) FI trigger points.

How about a fuel delivery problem? That could be another one...

dharder 09-23-2003 05:10 AM

I had a similar problem. It started out slowly and after a few weeks the car was almost undrivable. It would run fine for a while and then buck like a stallion. It turned out to be the FI points. The FI points tested fine but turned out to be junk. My car hasn't missed a beat since replacing them six months ago.

pbanders 09-23-2003 08:19 AM

Gerard, replace your trigger contact points if you haven't done so already.

3D914 09-23-2003 11:28 AM

Having the trigger points looked at today.

I'm curious though, wouldn't bad trigger points show up at idle and part-load also?

Another possibility I'm looking at is the coil.

Keep you posted.

pbanders 09-23-2003 03:58 PM

Coil was my other thought, too. The TC points wear so that they bounce at higher engine speeds, causing erratic injection.

3D914 09-23-2003 08:17 PM

Well the distributor checked out OK, and I installed a new coil. Still not working right. I'm wondering if the machine they use to check the distributor can apply an actual load, or is it in the form of RPM only.

I've got another distributor I will try tomorrow.

Enjoy! (well I'm trying)

Dave at Pelican Parts 09-24-2003 08:25 AM

The only "load" that affects the distributor in any way at all is the vacuum retard and/or advance. There are specs for that in the factory manual, for "mm Hg" (on the appropriate fitting of the distributor vacuum dashpot) versus degrees of advance or retard.


pbanders 09-24-2003 09:33 AM

Gerard, did you change the TC points yet?

robinaube 09-24-2003 10:41 AM

My car had similar bucking. My mechanic found a bad coil and brain. Once those were replaced, the bucking was less but still there. He found that the engine was lean at rpm range. He normally increases the fuel pressure from 29 to 32 psi but my car was already there. He recommended replacing fuel injectors.

My recommendation is to take your car to a mechanic with a fancy diagnostics equipment. You may have a lean mixture.


3D914 09-24-2003 02:10 PM


I guess they didn't replace them - they checked out OK.


In fact its running rich when bucking. I brought it into the garage - or should I say "wrangled it into the corral" - pulled the plugs (after a cool-down period) and they were black.

All injectors have been recently tested/cleaned (within 2 mos) and these are the best of the eight that I had cleaned.

In fact this all started 15 min down the freeway after picking it up at the shop. They fixed a pulled exhaust stud. They then checked the timing and idle with CO meter.

QUESTION: Does the fuel pressure reg mainly drop or increase fuel pressure from the fuel pump?

Quote from Apollo 13:
"Work the problem! Don't make it worse by guessing!"


Dave at Pelican Parts 09-24-2003 02:32 PM

The pressure regulator is a spring-loaded valve. When the pressure "upstream" of it increases past a certain point, the valve gets pushed open and pressure bleeds out and that fuel heads back up to the tank.


3D914 09-24-2003 08:01 PM

Thanks Dave,

So its a glorified relief-valve.

Well I've gone through a few iterations this evening and it seems to be running worse the more I check. I no longer have to load the engine, now it won't even rev over 3300 rpm without jumping all over the place.

My spare distributor runs about as bad or worse. I even tried a different cap and rotor. I'm going to have the shop checks its records and see if the ditributor rebuild included new FI points or not. If not, then they'll get replaced. Otherwise they're less than a year old, and I've only driven the car 6 mos out of the last year.

Also the fuel pressure was about 2psi off, but bringing it down to just under 30 made little noticable difference.

I did notice when I was checking connections on the relay board that the Volt Reg was VERY hot. Is this typical for running no more than 20 min or so?

Also, whats the best place to test the charging voltage, and the output from the voltage reg?

At the battery terminals I get 12.75 volts with no change up to 3000rpm. This is consistant with past measurements.

Enough for tonight.

pbanders 09-25-2003 07:00 AM

How did your TC's "check out ok"? Static testing is not sufficient, they must be tested while the car is operating. The only tester that I know that does this well is the Far West unit I have, and they're essentially unobtainable. You can use an oscilloscope to watch the contact closures to see if you get bouncing, but it's a difficult setup to put in place on an operating car w/o a portable o-scope (e.g. Fluke 97 or 98's).

I've found it's a LOT simpler to swap them out and see if it works. Let's face it, these are 30+ year old cars, you need to have a set of spare parts on hand for all of the FI components. If after swapping in, it doesn't fix the problem, no big deal - eventually, you'll need them. It's also a simple swap, takes less than an hour to do.

Dave at Pelican Parts 09-25-2003 08:35 AM

12.75V at the battery terminals is rather low. "Full battery" charge is 12.65V, so your charging system will not be recharging the battery very well.

I suggest going through the "charging system troubleshooting" tech article. It is possible that having low voltage on the battery/charging system is affecting the way the car runs.

From the "VERY hot" VR, I would suspect it--but something else could be causing that so it's probably a good idea to check things first.


D7N7MASTER 09-25-2003 03:17 PM

If you have NEVER replaced your F.I. points...the ones down in the bottom of your distributor, do this FIRST!!! Even if everything else in your fuel injection system is working properly, if the F.I. points have been fouled, you'll chase the problem. i.e. I cleaned the air cleaner, I (think) it ran better today...

3D914 09-25-2003 06:17 PM

Well I pulled the FI points to inspect them, and compared them to a good set that I had as a spare. I noticed when checking continuity that the outer spring on each side had somehow become "live" - so that when the point arm opened it could hit the outer spring and close the circuit again.

The spare set did not do this. The metal of the outer spring must be isolated from the circuit. Yet when I replaced them with my spare set - you guessed it - no improvement!

I think its time to order a set of carbs!


My car has never had voltage over 13. I've read some of these posts where guys are seeing 14+ volts, but my car has always run well under that.

3D914 09-27-2003 01:31 PM

OK I'm now dutifully embarassed.

The reason for the surging/bucking ended up being the tri-ground at the case. Even though it static tested OK, because of its location, I couldn't see that the nut on the other side had worked loose in less than 200 mi of driving. The whole thing was vibrating at RPM's - causing intermittent Injector connections.

So another problem solved, and two more discovered. I've replaced the injector points that needed it, and will now get to the alternator - since I'm only getting 12.5v. Plus all the connections in the engine bay and ignition switch are nice and clean.

Thanks & enjoy!

pbanders 09-27-2003 02:45 PM

Gerard, glad you found the problem, even though none of our advice was to the point. I'm surprised that this problem didn't manifest itself throughout all operating conditions.

3D914 09-28-2003 06:17 AM

Oh but it was!
We did identify the low-voltage problem. And I did get the trigger points replaced. I'll be installing a new alternator & VR early next week.

BTW - the current price of the $12 VR is now almost $60. This is becoming like owning a real "Porsche".

All times are GMT -8. The time now is 07:08 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.