# CIS engine with "Surging" RPM at idle and while driving! Help!



## nateturn (Mar 9, 2009)

I have an 86 VW Cabriolet with the 8 Valve motor and CIS injection system. It has a problem with surging RPMs (RPM not constant or smooth) when at idle and when driving. I don't know how the problem came about because the car was sitting for months before I got ahold of it. The idle is stable when first started but then starts to “surge”after about 3 minutes. At this time the idle seems to increase just a bit then it drops and increases again in a rhythmic pattern every couple of seconds. I have already checked for vacuum leaks as best as I could with a hose to my ear and couldn’t find any. If anyone has a better method or places likely to leak, I would like to hear it. I also checked electrical connections to make sure everything is connected (have very limited tools at the moment so couldn’t actually check for current). I checked and adjusted (centered) the air flow sensor plate but that didn’t fix the problem. 
One thing that I noticed while I was tinkering and trouble shooting was that the I could get the car to idle and rev smoothly if I manually connected the switch for the Full Throttle Enrichment (located on top of the throttle body). I am not even sure if the CIS injection system is supposed to have this component, but this car does have it. I read that this valve is supposed to increase the current to the Control Pressure Regulator (which is mounted on the front of the engine block on this CIS injected engine) to 14 mA. But isn’t the Full Throttle switch only supposed to function if it receives a signal from the Ignition Control Unit that the engine is above 4000 RPM? Anyway, I thought this was a good clue but don’t know enough about these engines to put my finger on the problem. 
Could the problem be irregular fuel pressure (control Pressure Regulator problem)? The Idle Speed Boost Valve? Throttle Switch? And does this model have an Idle stabilization Valve? and where would it be? 
What is the next Step? Anyone have any suggestions? 
Thanks,
Nate


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## WaterWheels (Aug 14, 2005)

*Re: CIS engine with "Surging" RPM at idle and while driving! Help! (nateturn)*

I think you are mixing two different systems here which might be helping to your confussion. I believe your car runs on CIS w/Lambda and some of what you are talking about pertains to CIS-e. The full throttle switch could very well be your problem, but I don't understand the way you explain what you are doing "...if I manually connected the switch for the Full Throttle Enrichment (located on top of the throttle body)". A bad O2 sensor could also cause the up and down engine speed as it continues to attempt to correct the mixture as could a small air leak. I would look in these three areas and while doing so find and purchase a book on Bosch fuel injection systems if you plan to keep the car.


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## 84 ROCCO 2 stroke (Nov 9, 2008)

my rocco does the same thing. And it is also CIS-lambda just unplug the oxygen sensor.


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## kamzcab86 (Feb 14, 2002)

*Re: CIS engine with "Surging" RPM at idle and while driving! Help! (nateturn)*

http://cabby-info.com/cis


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## bluestarcabby (Jun 2, 2008)

*Re: CIS engine with "Surging" RPM at idle and while driving! Help! (kamzcab86)*

Mine is an 89 Cabriolet and I have the same issue. I have unplugged the Oxygen sensor but that doesn't help. But if I squeeze the full throttle switch together it stops surging, or if driving and you let off the throttle it surges but if you step on the throttle it doesn't surge as much...
However, a few times after the car warms up it seems like something comes unstuck and it idles fine..maybe a valve is stuck open or closed?


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## redpig (Mar 29, 2004)

check for vacuum leaks, check your fuel dizzy settings


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## bubba_vw (Dec 3, 2008)

my jetta has started to the same thing


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## bluestarcabby (Jun 2, 2008)

*Re: (bubba_vw)*

Yes I checked the fuel distributor settings (hooked up a dwell meter it and can get it between 45-55 where my Bentley says it should be but still surges.) New oxygen sensor and fuel pumps last year.I have checked for vacuum leaks three times couldn't find any. The wierd part is sometimes (for a short time) it seems to snap out of it and idle fine-then it goes right back in to the surging thing....
I have checked relays and fuses.










_Modified by bluestarcabby at 8:24 AM 5-4-2009_


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## redpig (Mar 29, 2004)

I would change your injector orings, sometimes they harden and crack and cause a vacuum leak, even if you sprayed them to check for a leak, I would change them. also, make sure there are no rips or hole in your intake boots, take them off and inspect them very good, a pin hole will will make a difference


_Modified by redpig at 12:45 PM 5-4-2009_


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## bubba_vw (Dec 3, 2008)

*Re: (redpig)*


_Quote, originally posted by *redpig* »_I would change your injector orings, sometimes they harden and crack and cause a vacuum leak, even if you sprayed them to check for a leak, I would change them. also, make sure there are no rips or hole in your intake boots, take them off and inspect them very good, a pin hole will will make a difference

_Modified by redpig at 12:45 PM 5-4-2009_

i just put in new o-rings, new intake boots, O2 sensor, (fuel pump relay that was a month ago) less then a year ago. how offen do you have to change the injector holder?? would a dirty injector cause the problem also?

_Modified by bubba_vw at 9:31 PM 5-4-2009_


_Modified by bubba_vw at 1:48 PM 5-5-2009_


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## lilgreydentwagen (Sep 13, 2005)

*Re: (bubba_vw)*

check your isv (that little can with wires and vaccum hoses between the manifold and valve cover) when it starts to surge unplug the wire connector and see if it smooths out. if it does take the isv off and clean it out with some throttle body cleaner. if it's not that it very well could be the o2 sensor.


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## redpig (Mar 29, 2004)

I dont think 86 cabbies have an ISV


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## lilgreydentwagen (Sep 13, 2005)

*Re: (redpig)*

wierd. even my 80 caddy had one.


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## zcxerxes (Nov 13, 2005)

*Re: (lilgreydentwagen)*

my 86 cabby has a similar issue but only at idle. when in motion i have no issue whatsoever and it pulls harder than my girls 92 cabby and a buddies 92 gti. it seems that i've gotta adjust the air bypass screw with the climate change to keep it from stalling. but other than that it runs fine. i've looked and looked and looked but never found anything wrong


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## bluestarcabby (Jun 2, 2008)

*Re: (redpig)*

We had the injectors and orings replaced last fall.
The only oring I haven't checked is the idle bypass screw-
I have taken the intake boot off and inspected it a few times-no pin holes or vacuum leaks I could find. 
Replaced all battery grounds.
New spark plugs and wires
I will have to break out my Bentley and start testing everything-
The wierd thing is sometimes the surging stops and the car drives great, but only for one ride.


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## bluestarcabby (Jun 2, 2008)

*Re: (bluestarcabby)*

Found a leak in a fuel hose that goes to the fuel accumulator..
Will fix it this week and see if that solves our surging issue.


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