78SC engine problem - new clue
 

Newly purchased 78SC with 67,000 miles

Starts up every time (hot or cold).
Runs great if accelerating, coasting or idle.
Runs great until constant speed for a while (~2 miles) and it acts like it's out of gas.
It will continue to act like it's out of gas unless idle, revving with no load or above 6000 rpm with load.
Continues to act this way until idle or off for about 10 minutes.
Then runs great again.


New dist. cap, wires, and plugs.
Valves adjusted 6000 miles ago.
Have replace fuel pump, fuel check valve, and fuel filter.
Have checked cat. converter.
Have checked for rust in gas tank and lines.
Thought it might be exhaust gases leaking into engine compartment, but putting more fresh air into engine comp. didn't solve problem.
Thought it might be a clogged tank vent line, but running the engine without the gas cap has not solved the problem

New clue: problem above described with timing at 5 degrees BTDC.
With timing at TDC the problem deminishes, but is still there.
With timing at 5 degrees ATDC the problem almost completely goes away, but idle becomes rough.

Any guesses as to what might be wrong?

CHeck the electrical grounds. I had a similar problem that appears on a long road trip then it went away suddenly.


Noel

Electrical grounding problem, huh? Could you be more specific? I am very new to working on P-cars.

There is a few ground straps that I know of, There is one on the passenger side that is body to the trany, there is a other strap to the motor lid. I dont know of any others.
Other guys please chime in.

With all of the information listed above, this should be a well defined problem. I'm willing to give a $50 gift certificate to Pelican Parts to the person that knows what the problem is.

I have a similar problem on my 78SC motor in my 76S. I have been working on my car for 9 months!!!

Like you I have checked almost everything, but I get a sag and hesitation on very small throttle input underload, but go bigger throttle and it takes off like there's no tomorrow. I have checked the ignition numerous times at 5 deg. ATDC and it advances as it should witth RPM. Have KV85 wires and it helped a bit, no more backfiring, but still sags.


So I don't have a solution for you, but please share if you or someone comes up with a solution. It may be the cause of my problem too. I'll even put match your $50 Pelican Gift Cert for an answer that "leads to an arrest" of my problem.;) :confused: :mad:

I'll let yo know if I find anything. Good luck.

I had a 68 bug that I was having a similar running problem after jumping through hoops and changing parts I was down to looking in the gas tank, what I found was a piece of small plastic that was getting sucked into the outlet after a few miles. Might be a long shot but you never know. Take off the cover to the full pump and remove the pump and take a look.

Thanks GB, but the plastic in the gas tank doesn't jive with the pick-up at 6000 rpm every single time.

This problem is so consistent, that it's probably a part failure, not a random occurrence.

Razor....just some thoughts...

Your clues are not entirely clear , but let me give it a shot:
I believe its either ignition or fuel ( not likely mechanical or a 'hard failure') so let's look at ignition first: pull the dist cap and turn the rotor - it should turn one direction (counterclockwise) about 10 degrees, and then 'snap' back when released. If it sticks, or goes back slowly, then you've got some crud in your centrifugal advance, and you need to disassemble/clean the distributor's mechanical advance mechanism. (note: I don't think this is the problem, but since you reported that you had some improvement with timing change, I thought this may be worth a look) When youre' done, put it back together, and check to see if you have centrifugal advance, by running the engine, and blipping the throttle - with a timing light hooked up, you should see the timing mark jump (advance) and come back to the same place, when the trottle is released.
Part 2: Fuel - remove the rubber bellows from the intake, and gently lift the air flow sensor plate (I do this with a small magnet - on the bolt) The plate should move up and down smoothly, with no resistance. It's possible that there is a burr or rough spot on the arm, causing the plate to "pause" . Also, look down into the plate housing, to make sure that the plate is concentric with the hole - if the plate rubs on the side, it will 'stick' at certain points.

Good luck - let me know what you find.
j.

J man,

Thanks for the good feedback. I think we are getting warm.

Part 1: Looks good. The dist cap goes about 5-10 degrees counterclockwise, then snaps back. The timing also does advance upon higher rpms.

Part 2: Where is the rubber bellows at the intake? Also, since I have not seen the suspected valve, what size bolt / magnet should I get?

The mechanic who tested / passed emmissions also suggested the air flow sensor or flow valve.

Looking forward to your reply.

Razor

Parts...
 

Razor:
look at this page (pelican diagram)
http://www.pelicanparts.com/911/911_Parts/1978-83/1-7.JPG
That will give you the bolt (part #13) and the arm (#8) The bolt is the one that I use to "pull" on. It holds the round air flow plate to the arm...as air flows up, past the plate, the force of the air on the plate lifts it, and increases the fuel supply, to match the increased air flow.
On this page:
http://www.pelicanparts.com/911/911_Parts/1978-83/1-7-2.JPG
you can see what I called the "bellows" which is just a rubber connector to the air flow valve housing. It's part #18. Just loosen the #19 and #20 hose clamps, and lift the rubber thing up. You will then see the air flow plate and the bolt.
(no need to use a magnet - you can just use a pair of pointed nose pliers and gently pull up on the arm, to see if it's moving smoothly.
Remember to check: smooth operation, plate concentric in the round housing, no apparent damage to plate (it should be smooth and flat)
If you need more info, the Bosch injection system is not special to the Porsche - it's the same as the similar vintage Saab, Volvo and others.
Bosch has plenty of info on the system, including curves for pressure vs air flow and other stuff that only rocket scientists could measure ! Anyway, what's important is that it LOOK and function correctly. As I recall, there is a "practical"test you can run, where you pull the injectors out and activate the fuel pump and lift the air flow plate - when that happens, you get a high pressure shot of fuel for about 1-2 seconds, and you can look at each injector's spray pattern, etc.
Mucho caution, though....the fine mist that is sprayed by each injector is highly flammable, and you must be very careful to have disabled any spark source!
Good luck.....let me know when you kill this gremlin!

J-man,

While taking the rubber boot off, it ripped. Maybe it already had a rip and I just aggravated it. Anyway, I put some silicone rubber on the crack and it started up. I think if I drive it any distance, the silicone rubber will melt and I could be stranded.

Pelican Parts is sending me a new one on Saturday (I hope).

The disc had some black crud on the top and sides, but the disc seemed round and the crud didn't seem to interfere with the disc.

I will take it out for a short test drive now...

...back from the short test drive.

It started out smooth, but after about 1/2 mile acted worse then ever. It acted like it was going to die out completely. I thought the silicone rubber had melted but when I got home, it was cool and in place.

Could this black crud that I wiped off be causing the problem? Maybe it's gotten worse because I smeared it around. Or could it be a vacuum leak? I tried spraying carb. cleaner, but couldn't tell if there was a leak or not.

We are getting real close. I can tell. It's either the plates you have described or a vacuum problem.

Looking forward to your response.

Razor,
Hmm....if the rubber was bad...that could be a big part of the problem. For the injection system to function properly, there must be an air-tight seal between the air flow valve, and the throttle. Sounds like you've got some leaks, if you found a crack.
Possible scenario: cold, the rubber is stiff enough to keep the leak to a minimum - but as the car warms up and the rubber softens, any crack would grow, as the rubber would begin to yield.
Of course, as I think this through, it does NOT explain why wide open throttle (WOT) gives you plenty of "go".....hmmm have to think some more,....
Since the large rubber boot or bellows (gee, I"ve g
ot to get the right name for that thing) is bad - and I can tell you that it's generally pretty high quality rubber in those things - I would be very suspicious of any rubber part (hose, seals, etc) under the hood....you should take a hard look under there, for cracked hoses and the like.
Sorry I can't help more - I'm in Ohio and I see your a Texan (so's my wife!)
By the way....you did not say if the air flow plate moved up and down smoothly or not....this is critical... be sure to check it carefully (and while I'm thinking, you should try to check it warm...since cold, your engine does not exhibit this problem) Perhaps you can run the car with the "old" boot (see I looked it up!...hooray, now I know the name) and then when it's warm, remove the boot ; check the air flow sensor; if all is well, then put the new boot on and test drive.
I notice from the pelican parts listing that there are MANY hoses in and around the boot....do a careful inspect on these- pull each one off, and look at the spots that typically go bad : that's the first inch or two , where the hoses attach to something, and at each bend and stress point. Rubber does not like to be in tension - or stressed. (sorry for the lecture - I work for Goodyear and I do this stuff every day)

OK...'nuff said - let's see what's going to happen with the new parts .....
(fingers crossed)

Could Razor need a new ignition coil? Just a thought, as he did not replace it yet. Could the coil's performance deminish at the higher RPMs? I saw a post on Rennlist with a reponse that a new coil fixed a poorly running 3.0.

Might be worth replacing anyways since it is less than $100 and may be as old as the car.

I installed a 3.0 that had quite a lot of oil crude in the intake. I cleaned everything I could get to w/o removing the airbox. I can see the same crude downstream of the throttle body (I didn't think to clean it all up when the motor was out of the car, doh!. Might have to pull the motor if I can't get it to run normal). Seems like Razor's motor is/was in the same condition as mine was. And the point is...I can get my motor to spool up to redline with ease. So I do think you 6000RPM problem is crude related.

Souk
(w/ a 3.0 that is not quite running right yet, still!)

Thanks for your support J-man and Souk. I'll know more by Monday. I might call tomorrow and add an ignition coil to my Pelican order.

[QUOTE]Originally posted by Souk
[B]Could Razor need a new ignition coil? Just a thought, as he did not replace it yet. Could the coil's performance deminish at the higher RPMs? I saw a post on Rennlist with a reponse that a new coil fixed a poorly running 3.0.

Might be worth replacing anyways since it is less than $100 and may be as old as the car.

Souk,
While I could be wrong, I don't think that Razor's symptoms would "fit" with a bad coil. Usually, a dying coil will exhibit breakdown at higher engine rev's...due to the shortened time between spark pulses. Razor says that his car goes like a bat, at WOT, but is weak in the knees at part throttle....
These 'clues' tend to point more towards fuel/air problems, rather than coil. (but hey....I'm 1500 miles from his car, and am just making educated guesses!)
joeycatt.

Joey, you may be right about the coil, but if the coil is old, why not replace it. Also, electrical problems can cause mysterious symtoms.

I have the problem with part throttle (up to about 10%, big guess at %). Razor has a problem that develops after cruising at constant speed for a period of time.

Mysterious! My 78SC motor is supposed to have the same mileage.

OK, I'll bump the Pelican gift reward to $100 for the person who can solve the mystery. (I'm replacing my coil so that solution is out).

Purchased Bosch's Fuel Injection Systems book today. This should help provide a bunch of clues. But my boot didn't arrive yet from P-parts. :(

Well the new boot arrived and it didn't solve the problem.

I thought I had the answer when I found the vacuum hose to the spark plug distributor was plugged. I purchased a new vacuum hose, but the problem persists.

Here is another explaination of th problem:

Starts great hot or cold.
Idles great and revs great with no load.
Hesitates under load until the rpm's hit 5800.
Heavier loads generate a stronger hesitation. (For example, decelerating in 2nd gear - no hesitation. Accelerating in 3rd gear, more hesitation.)
Runs great under load with rpm's between 5800 and 7000.

Razor, joeycatt made the statement that "usually, a dying coil will exhibit breakdown at higher engine rev's...due to the shortened time between spark pulses."

I still think you should look into a coil replacement first. Then try you distributor.

Joeycatt, if the coil is good, could his distributor be binding at higher revolutions or are the counterweights not moving freely thus not providing the correct advance at high revolutions?

Souuk H
(Still itching to take my distributor off and R&R)