![]() |
|
|
|
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
hesitation/slight missing while cruising
Hello,
I have a problem while cruising at a steady speed most noticeable around 2500-3500 rpm. Its like a couple second hesitation or slight miss, I don't think I would describe it as a surge (but I could be wrong). Engine pulls strong all the way through if I am heavy on the throttle and doesn't seem to do it over 4000 rpms if just cruising. The tach doesn't move, it will remain at 3000 or whatever rpm I am cruising at but you can feel the missing/hesitation as a vibration. I have been trying to fix this problem by adjusting the mixture by ear without any luck. It idles fine at around 950 rpm. I just re-checked my fuel pressures and they seem off from the info I have found. System pressure with valve closed and wur unplugged 6 Bar Valve open with wur still unplugged pressure drops to 1.3 Bar Plug wur in and pressure slowly rises to 3.8 Bar Engine warm with wur plugged in and valve open 6 Bar Any suggetions appreciated The Car is 1978 ROW 911SC with 964 cams Thanks Craig
__________________
1978 RUF 911SCR |
||
![]() |
|
ROW '78 911 Targa
|
When was the last time you changed the fuel filter?
Your pressures don't seem to be too far off. How cold is it when you are measuring the WUR cold unplugged pressure. (Seems a little low) Have you lubed the distributor lately? That is about the rpm range for the advance mechanism to start kicking in.
__________________
Dennis Euro 1978 SC Targa, SSI's, Dansk 2/1, PMO ITBs, Electric A/C Need a New Wiring Harness? PM or e-mail me. Search for "harnesses" in the classifieds. |
||
![]() |
|
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
Fuel filter is new. I have never lubed distributor, any tips?
I was thinking system pressure should be between 4.5-5.2 and I am at 6 bar? |
||
![]() |
|
ROW '78 911 Targa
|
To lube dizzy PPP off rotor and drip some 30 weight oil on the felt. Get it saturated.
Not sure on the fuel pressure being high, restriction in the return line? |
||
![]() |
|
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
Ok, I'll give that a try.
Thanks |
||
![]() |
|
Registered User
Join Date: Aug 2013
Location: Whispering Pines, NC
Posts: 333
|
I had a very similar thing and it turns out to be an intake gasket leak. when I get the car back I will let you know if it is fixed.
__________________
1985 911 Targa 3.2 with 964 Body Kit |
||
![]() |
|
![]() |
Must go faster...
Join Date: Nov 2008
Location: Bainbridge Island, WA
Posts: 125
|
I have been going through a similar thing with my '78. but it was more than a hesitation. I would loose power then a huge backfire! Today, I just change the CDI over to MSD and so far have seen a great improvement. Not sure if this is similar to what you are experiencing but that could be the beginning of the CDI failure.
Just a thought. |
||
![]() |
|
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
Ok I lubed the distributor. I will also double check the intake runners as I did have the cis off recently.
|
||
![]() |
|
Registered
Join Date: Jun 2011
Location: Placerville, CA.... You know, the only place on Highway 50 between Sacramento and Lake Tahoe the you find signal lights. Above the fog most of the time and I can see the stars of the Milky Way 8 out of 10 nights. Kinda cool.....
Posts: 6,329
|
My 78 SC is at a local shop for this exact same issue. Grenades the distributor last July but that was replaced. This mysterious hesitation/stumble/miss or whatever you would like to call it has the techs stumped. CDI box swapped, wires, harness, etc has been checked with no luck.
I will mention in intake gasket possibility. Very interested to hear what everyone's own experience will be regarding the fix for their cars. Cheers
__________________
"What the hell is an Aluminum Falcon??" -Palpatine (Robot Chicken) 1978 911SC Targa Working Projects: 1968 912 |
||
![]() |
|
Enthusiasm > Expertise
Join Date: Mar 2011
Location: Outside Philly
Posts: 606
|
When did this issue arrive? Any idea why? What other changes have you made, if any?
I like your thinking -- I'd start by fixing the fuel pressures and move on from there. I would dial the system pressure to 75psi by playing with the shims on the regulator. Do you have a copy of the Probst book? Too much fuel pressure is just as wrong as too little. |
||
![]() |
|
Registered
Join Date: Jul 2001
Location: Portland Oregon
Posts: 7,007
|
I'd be checking AFR's (CO%) at cruise RPM to see if you have a fuel or ignition problem.
What plugs are you using? How old are your plug wires, cap & rotor?
__________________
Steve Weiner Rennsport Systems Portland Oregon (503) 244-0990 porsche@rennsportsystems.com www.rennsportsystems.com |
||
![]() |
|
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
I just reinstalled engine after having top end apart to replace lower head studs.
Cap and rotor are about a year old, the plugs are new Bosch (don't have the number in front of me), wires were replaced about a year and a half ago |
||
![]() |
|
![]() |
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
I also replaced all the injector o-rings and sleeves and intake runner rubber and gaskets
|
||
![]() |
|
Registered
Join Date: Jun 2011
Location: Placerville, CA.... You know, the only place on Highway 50 between Sacramento and Lake Tahoe the you find signal lights. Above the fog most of the time and I can see the stars of the Milky Way 8 out of 10 nights. Kinda cool.....
Posts: 6,329
|
Do you have any additional updates of your issued?
__________________
"What the hell is an Aluminum Falcon??" -Palpatine (Robot Chicken) 1978 911SC Targa Working Projects: 1968 912 |
||
![]() |
|
Registered
Join Date: Oct 2006
Location: MYR S.C.
Posts: 17,321
|
3.8 is on the hi end and 6bar is hi. these both will lean the mixture.
the 2500-3k miss on the CIS 3.0 seems to be a common problem, especially when something is not right. air leaks is first thing. intake bolts, vacuum lines are some things to check. you also have the vacuum enrichment WUR. check the control pressure with vacuum (about 3.6bar) and without vac( about .8 bar less). then its a matter of mixture and timing. try moving the timing up and back 5 degrees and see if it gets any better. richen the mixture as much as the idle will allow. IE. keep adjusting it richer until at least one of these 2 things happen. it surges at idle or when you rev it up and let the idle come down, it dips down below idle then comes back up. you want it right on the edge of this happening.
__________________
86 930 94kmiles [_ ![]() 88 BMW 325is 200K+ SOLD 03 BMW 330CI 220K:: [_ ![]() 01 suburban 330K:: [_ ![]() RACE CAR:: sold |
||
![]() |
|
Registered
Join Date: Oct 2012
Location: Chilliwack BC, Canada
Posts: 475
|
Problem has been solved!!! It turned out to be a partially clogged injector on cylinder 4. It was spraying somewhat sideways and seemed to require some addition pressure than the other injectors to spray so I am assuming that cylinder 4 was missing when heavy throttle was not being used.
I tried cleaning it with some carb cleaner and compressed air but it didn't work. I replaced that injector with a new one and the hesitation/miss is gone. I agree the fuel pressures are still higher than usual but the car seems to be running good now. Thanks for all the suggestions. I am slowly getting to know my engine pretty well ![]() |
||
![]() |
|
Registered
Join Date: Jun 2011
Location: Placerville, CA.... You know, the only place on Highway 50 between Sacramento and Lake Tahoe the you find signal lights. Above the fog most of the time and I can see the stars of the Milky Way 8 out of 10 nights. Kinda cool.....
Posts: 6,329
|
Congrats on locating your fix. Will need to check out the same on ours. Will need to check for possible air leaks and/or blocked injectors.
Cheers
__________________
"What the hell is an Aluminum Falcon??" -Palpatine (Robot Chicken) 1978 911SC Targa Working Projects: 1968 912 |
||
![]() |
|
Registered
Join Date: Jun 2011
Location: Placerville, CA.... You know, the only place on Highway 50 between Sacramento and Lake Tahoe the you find signal lights. Above the fog most of the time and I can see the stars of the Milky Way 8 out of 10 nights. Kinda cool.....
Posts: 6,329
|
Her is one for the "strange and unusual" regarding these symptoms.....
Our 78 911SC we're exhibiting the exact same symptoms. The cause of this great mystery...... The EGR valve. Who would of thunk. Just one more thing to check when trouble shooting this type of issue. Cheers Trevor Sent from my iPhone using Tapatalk
__________________
"What the hell is an Aluminum Falcon??" -Palpatine (Robot Chicken) 1978 911SC Targa Working Projects: 1968 912 |
||
![]() |
|