[Discseven]

Originally Posted by firstclassof86

I had a bad fuel damper. Started with bad fuel mileage. Progressed to rough idle and then smoking( running rich). car relies on vacuum at lower rpm to pull fuel. Vacuum is highest at idle and lower at higher rpm. Could explain the mileage difference when car is operating at lower rpm(higher vacuum)during city driving. If you have a small tear in the diaphragm in the fuel damper, it will pull unmetered fuel into the intake manifold . Just a guess on your issue. maybe test the fuel damper?

86... are you referring to the dampener on the 4-5-6 rail? If so, to test for a tear, is that the check for fuel in the vacuum line connected to it?

Originally Posted by 917_Langheck

Need a friend with a known good 3.2 that passes smog and swap both DME and AFM from that car to yours. I don't think you have adequately ruled those out at this time. I like the fuel damper idea too. Don't remember from the other thread if that was looked at it not.

917... I like that idea. No one I know locally has a Motronic ECU / AFM. You are correct about neither being ruled out. Would cost me $460 to "rent" an ECU from Specialized. ECU is a point of special interest given it's history as I noted in the first post. AFM... will learn exactly what adjusting the spring tension and wiper arm adjustments do. Then see if anything about those areas relates to this.

Originally Posted by Mr. Merk

This is wild—my '88 consistently gets over 20 mpg.

In my experience, it’s rarely a coincidence when something changes. It almost always traces back to the last thing that was altered or disturbed.

Something worth checking: I recently learned about a potential vacuum leak source. The thermoswitch on the back of the throttle body controls the ambient air valve that draws "warm air" from the oil tank into the throttle to prevent icing in cold temps. If this system fails, it can also lead to increased oil consumption.

I plan to eliminate that system entirely. It might be worth ruling it out on your end too—though I doubt it would account for such a drastic drop in fuel economy.

http://forums.pelicanparts.com/porsche-911-technical-forum/987569-ambient-air-valve.html

Merk...



I believe those caps are tight but will check them today. That switch has been capped for many years---when engine was running 18 city mpg.

Originally Posted by Monkey Wrench

ive had an injector stick open then it would dump fuel into the cylinder bore whe parked , and blow white smoke upon startup. it wasn't caused by a new pope ;-) I think you would have caught that if it was the cause.

I was just reading anbother post referring to the coil, and it went into depth explainng how the coil charged and how it has less time to charge at higher RPMs , the factory engineers probably did not make bad decisions on the ratio of the windings etc but some coils ou there seem to perhaps a bit falsely adverticse that swapping to their new fangled coil with a different ration , perhaps a higher output voltage , imporoves the spark.

it seemed to suggest that installing a "hotter" coil could actually cause the transistor in the ECU or I shoudl say MCE to degrade and that the programming may be adapted change this charge rate. I wondered if perhaps a higher RPMs if there was misfiing going on that wasnt; that noticible?

it made a valid point that at high rpms the time for the coil to charge for it's next spark is shortened and that would make the spark weaker.. and I assume a coil with a different winding ratio may influence this.

I dont have the theory to back all that up nor the experience. im as confuesed as anyone about your real cause of the poor mileage. Nothing I said above was said with a wealth of experience under my belt so take it with a pinch of salt.

i wonderd if you had updated to a coil with a different winding ratio and if swapping to the OEM one might affect the mileage?

obviously driving habits are an influence in mileage so in the city if you are using wide open throttle well That could be quite habitual for some and "never" for others. eg some will go wide open throttle when the next light is red, because they are enjooying doing so mostly,

Others think ahead and want o save fuel and may think well im going fast enough to coast to the light before it changes so why would I apply any more throttle? Ill lay off and allow my momentup to get me there in time for the light ot change because anything other is just a waste of gas.
some may even change their pace to avoid stopping and starting as each start uses fuel , truckers are masters at this. they will percieve the other light to predict more closely . when its going to change.

no matter of your personal habits you are probably still comparing oranges to oranges because any changes will reflect the same driving habits.

Wrench... Blaster coil went in with the rebuild. I checked specs before doing this and on paper, Blaster matches the Bosch (black) original coil. I got a used black Bosch from eBay on Sal's recommendation to swap it for the Blaster. eBay coil was no good. Am not to fond of eBay. Doing that again... little appeal. But, Blaster coil, like the ECU, were not with the engine when it was doing 18 city. So Blaster is added to the culpable list together with the ECU. I will look for a used black Bosch on Pelican.

With city mpg as it is, am driving very conservatively. I do look ahead, coasting when possible. And timing traffic lights. Were the engine doing 18 city, I might get 20 in let's call it Lindbergh mode! Something to test after the current fuel puzzle is solved.

Originally Posted by Otter74

Given all the data you've collected without any obvious clues, and given that the ECU is not the same, if you have the ability I'd try to borrow another ECU to swap and test the car with that.

Otter... yeah, I may get to renting one from Specialized. Don't know anyone local to do this with. Am hesitating going the rent-an-ECU program on the basis that the ECU has been tested. Doesn't guarantee it being 100% good. ECU is on the "culpable list" together with the Blaster coil.
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