[boon94]

So i've been getting rather poor mileage lately.. cant seem to get more than 23 - 24 mpg (all highway driving at 75MPH... usually get 29-31 MPG )....

So since i've been getting horrible mileage, i said, well im not wasting 93 octane super premium on this (different octane ratings in europe before this conversation starts again).... So i put in the regular 87 octane (my tank was empty)... So after driving for about 10 minutes i was about to accelerate from a red light and when u put 1/2 throttle down and it boosts to top ish rellow area (not even full boost like if i was flooring it) the engine made a shudder and knocking noise (almost like in the movies when a car runs out of gas) and the boost cut down instantly, and my acceleration was sloooow.

Driving with 93 octane, sometimes my boost gets stuck in mid yellow when i floor it in 3rd or 2nd gear, but lifting then re-flooring usually allows full boost. I thought the APC was just bad until it started getting so much worse with lower grade gas.

Flooring is in 2nd and third with 87 has really bad knock so i decided i better disconnect the APC and run on base boost. (its not really been knocking on base unless its just comming from a start and im not giving it enough gas).


Whats causeing my engine knocking? I dont even understand why this would happen. Faulty oxygen sensor? I've had my engine re-built roughtly 10,000 miles ago, not leaking oil or coolant. Doesnt get better or worse depending on engine temp.

My exhaust is rattleing really bad . Is there a sensor in the exhaust that could be throwing something off, or getting thrown off from a bad leak?

I dont expect it just to be a bad batch of gas since i've been noticing the problem ,and its just been getting worse with time (as has my exhaust leak)... And i'm on my second load of 87 octane

 

[Furious_Gopher]

I have the same problem; Awful buckling with even the slightest amount of boost sometimes.

I want a sure answer too.

 

[Thirsty9000]

I'm assuming you've tried new plugs and DI Cassette. You say your engine was rebuilt semi recently, is the timing proper? These are the first thing's I'd think of to check. 23 mpg is as good as I ever get from mine in mixed driving. 25.5 is as good as I get on road trips if I'm conservative with the cruising speed. Your mileage doesn't seem that rediculous.

 

[boon94]

Im trying new plugs when the rain stops, i think that will do the trick.

Do plug gaps seem to increase with age? My indy gave me a set of the NGK for $9.00. My DI casette has 15,000 miles on it, its less than a year old

If that doesnt do the trick, how would one go about seeing if the timing is right? Is this a mechanic type thing? or easy to check myself

23 mpg is as good as I ever get from mine in mixed driving. 25.5 is as good as I get on road trips if I'm conservative with the cruising speed. Your mileage doesn't seem that rediculous.

Yikes

 

[boon94]

ok I finally got around to changing them today and it made a huge difference.

The old spark plugs were kinda rusty, and gaped at ~1.6 mm.... which is way off, 50% larger than its supposed to be.

I got the new ones and they were factory gapped at ~1.4 mm and i was supprised because i had read that the NGK's were factory gapped at the proper spacing for saabs, which they clearly are not, maybe for a non turbo?

So i lowered them to .9mm (~0.035 imperial), used some grease stuff and ran it though the adaptation run, and my boost is perfectly smooth, I can get full boost in 3rd and 4th gear from 1800 rpm to 5000 rpm with no bouncing or fluxuation now, it runs very smooth!

Im glad it was so easy.

.. And my mileage is back up to normal, hitting 31 average on the highway again