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BlueQuestTSi's Avatar
 
Join Date: Jun 2003
Posts: 396
Fuel economy issues..

Ok. I only get about 22mpg Highway. This issue has plagued the car since I've brought it home, but I wasn't concerned, as I felt once I start replacing items I'll find and resolve the issue.

So far :

New Plugs and wires. Old wires were severely cracked.
New Timing/Balance shaft belts.

Checked/cleaned Distributor cap and rotor.
Airfilter is good relatively new, clutter free.


Not touched thus far:

O2 sensor < --- Top runner on my list.
Fuel filters.
Haven't cleaned or run cleaner through injectors.


I'm not sure that I have an overly rich condition, as I smell no gas, and haven't seen a smoking issue. But I might have to have the wife follow me to verify this.



The only driveability issue with the car:

After full to 3/4 throttle acceleration, passing, and 'foot off the gas pedal' deceleration, I get a miss when I level out to my cruising speed(when slightly re-opening the throttle). Almost feels to me that the car went overly rich upon decel, and it seems to sputter for 15-30 seconds then levels out to a normal run.

Other wise, the car idles very smoothly, and accelerates smoothly. There seems to be a slight dip in power right at about 4krpms. But otherwise, it purrs and runs like a kitten.



Suggestions, thoughts, criticisms?
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Jack
86 Red 951 - Turbo Twists, otherwise stock for now
83 Black 944 w/86 951 front end - still down and out, but coming back slowly.
02 Altima SE 3.5 - grocery go-getter
00 Yellow Ducati Monster 900i.e. 88 Chrysler Conquest TSi
Old 10-04-2003, 03:57 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
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Join Date: Dec 2002
Location: Richmond, VA
Posts: 484
Jack:

I had similar fuel economy issues with my '84, as well as a pronounced "stumble" or hesitation near 4K. This hesitation manifested itself as though a REV limiter had kicked in. If I feathered the throttle slightly, the engine would pick back up and the car would accelerate normally.

Based on the fact that the hesitation was more pronounced when the electrical system was heavily loaded, coupled with the fact that the engine passed NC emissions standards with flying colors (The inspector remarked that this engine runs cleaner than most new cars.), I concluded the problem was electrical in nature.

I went through the plug/wires/fuel filter/FPR/clean injectors routine, and even resorted to tweaking the AFM internals, but I only made minimul progress, so I shifted to fixing the hesitation issue.

What I discovered is that by replacing the DME and AFM with parts from an '86, the hesitation disappeared, and my gas mileage improved by about 4 MPG (City/Highway).

I suspect the original DME is suffereing from some component aging. As the electrical system is loaded, the over all system voltage may be dropping enough to affect the internal reference voltages that the DME uses. Since the references are inaccurate, the DME will give an inaccurate out put.

Part of this reasoning stems from the fact that the early DME sends an un-regulated 8 volt reference signal to the AFM, while the later DME sends a regulated 5 volts. With an un-regulated reference, a drop in the source voltatge will cause the reference to drop, which in turn affects how the DME interprets the difference between the sensor out put and the reference, thus causing errors. This situation is worsened by the fact that there is very little room (4 volts for an early DME as opposed to 7 volts for a late DME) between the reference and the source voltages.

Making the swap is pretty painless. You don't need to modify the DME harness, just take out the original parts and put in late model parts.
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Cliff Hipsher
'87 944 India Red
'86 951 Kalahari Metallic
Old 10-05-2003, 09:23 AM
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BlueQuestTSi's Avatar
 
Join Date: Jun 2003
Posts: 396
Quote:
Originally posted by Porsche944
Jack:

Making the swap is pretty painless. You don't need to modify the DME harness, just take out the original parts and put in late model parts.
Painless sounding enough. Just a matter of finding some good parts at a reasonable price.

That's also a part I had considered, considering the over-all simplicity of the engine.

Thanks.


Any one else have suggestions?
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Jack
86 Red 951 - Turbo Twists, otherwise stock for now
83 Black 944 w/86 951 front end - still down and out, but coming back slowly.
02 Altima SE 3.5 - grocery go-getter
00 Yellow Ducati Monster 900i.e. 88 Chrysler Conquest TSi
Old 10-05-2003, 10:25 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
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