what should i try next?

a while back i posted that i was experiencing some pulsing/hesitation in lower rpm's on my car. it smooths out at about 4000 rpm, regardless of how hard i drive the car. since then, i've changed the plugs, distributor cap, and rotor. still no change. i guess i'll swap the o2 sensor next... can anyone offer any suggestions for me?

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Adam Nitti
www.adamnitti.com
'85 Carrera
'74 3.0 CS

When my butt feels surging (usually pretty subtle), or when the car hesitates or gags, I suspect a possible vacuum leak. If air is getting into the engine without going past the air flow sensor (or any other device which is supposed to pass air), then the car will not run right. Generally speaking it will run too lean, which is a result I am scared of.

Dirty injectors can also have these (surging and hesitating kinds of) effects.

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'83 SC

ok, thanks. but where do i start looking for the most likely vacuum leaks? looks like my bentley manual has a method for testing injectors, in the meantime.

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Adam Nitti
www.adamnitti.com
'85 Carrera
'74 3.0 CS

I have a hard time finding vacuum leaks, to be candid. Spraying cab cleaner around the intake manifold or plenum, they say, can identify them, but I've not had good luck.

Listen for whistling. Cars that whistle, by the way, are often driving with serious intake leaks. Spraying water sometimes works. Both water and carb cleaner can change the engine speed when sucked into a leak. spraying into the air intake will do this too, so you want to spray without getting it into the (appropriate) air intake location.

Can someone else tip us off as to an awesome method of finding intake leaks? Perhaps starter spray (ether) would make these leaks obvious.

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'83 SC

Adam,
I'm not too sure whether either of my suggestions will work or is the culprit, but you can give it a shot. As Superman mentioned, it could be dirty injectors. Just dump a bottle of fuel injector cleaner before you fill up and see if it makes a difference. Also you could check to see if your mixture is right or not. A lean mixture will cause the hesitation/pulsing, at least that's what I experienced when I was running lean; hesitation at low rpms and smoothing out as rpms climb. Hope this helps.

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Jeff
1976 911S

Ruf is right, as usual. I said "Ruf," not Rush."

Lean mixtures can cause surging, popping, all kinds of offensive stuff.

I have been told that the "ChemTool" stuff (I think there's a numerical component to the name of this stuff) is the best for ce=leaning injectors, and I also be lieve it is recommended by all German car makers. I have also been told that you can wait until you tank is only 1/4 full, pour most of a bottle in, drive it a day or two, then fill up and add another bottle. Check it out carefully, but I was told these stuff like this by persons who are fairly respected for their understanding of german car mechanix.

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'83 SC

Adam,

B-12 ChemTool is available in both spray cans and in liquid form for pouring as recommended above. The spray cans come with a long plastic tube that can get down close to your manifold mating surface at the individual head intake ports to check for loosened manifold nuts. If you spray all around each manifold runner at the flange, any vacuum leak will cause a change in idle if B-12 is sucked into the port ...

If you have or can get a small jewelry-type ultrasonic cleaning machine, you can clean the individual injectors by soaking overnight with liquid B-12 ChemTool in the ultrasonic machine, after you have removed them from the engine. If the ultrasonic machine has a plastic tank, be sure to protect with a makeshift 'pan' of heavy-duty aluminum foil, as the B-12 can attack some plastics.

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Warren Hall
1973 911S Targa

I understand the 84-89 911s tend to get cracked intake manifold gaskets.

The gaskets from the riser to the head.

Good luck
Nick

'85 911 pops and misses when hot.

I thought we'd hear from Warren on this one. When my engine was out and I changed the injectors, I also changed the intake runnder things (short rubber tubes connecting the runners to the air box) and the gaskets between the runners and the head. The tube things were visibly shot (bulging like old coolant hoses). I'll try the B-12 ChemTool test for leaks next time I try.

My fill-up was 17.362 gallons at 386 miles (22.23 mpg). So you see, I could get 450 miles. I have to confess however, that I use supreme unleaded (Chevron) exclusively. I'm not sure this affects mileage but it could. It DOES help ensure against detonation.

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'83 SC

How about a new fuel filter?

just picked up some of that b-12 from the auto parts store. they didn't have the spray, though. is it worth it to try it as a fuel tank additive first, or should i just bite the bullet and remove the injectors?

new fuel filter? it's worth a shot. guess i'll try that, too. have no idea the last time it was changed. i've only put 1300 miles on my car since i got it, since i'm constantly on the road.

thanks everyone for the feedback so far.

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Adam Nitti
www.adamnitti.com
'85 Carrera
'74 3.0 CS

I can't remember exactly where I heard or read it, but I recall that mechanics usually use propane from a tank to locate vacuum leaks. If you have a can (same as for a barbecue grille), attach a hose and a thin copper pipe to it so you can reach around various suspect places on the engine... the induction of the propane thru a vacuum leak should cause it to change idle when the leak is located. Try a bit on the air intake first to discern what kind of an idle variance you should be experiencing if you've located the leak. (I'll bet, however, that its either a dirty gas filter or dirty injectors... try a bottle of Techron for the injectors)
regards,
jlex.