Please Help. Sudden Huge MPG Drop After Mechanic Worked On ...

Would appreciate any guidance on diagnosing an overnight drop in MPG from 20 to 12 after my mechanic replaced a few leaking fuel injectors and follow-up
work after I reported no pickup, putt-putting, excessive exhaust, and no power/acceleration. It turns out there was an electrical problem with one of the new injectors, which he "made something" to correct. However, instead of focusing on the few new injectors first, I've got new rotor, dist cap, plugs, and wires (some which look too long) and 1/2 the MPG. To make matters worse, the original problem reoccurs periodically. Now that it's colder out, it's occurring more often. I'd like expert opinion so I can try correcting it myself and/or go back loaded with "pre-diagnosis" if possible.
1984 Maxima 5spd, straight-6.
Other symptoms ... (1) When starting the car, the RPMs would start high (around 1700 I think) and gradually lower to a consistent point as the car warmed up. That no longer happens. (2) The warm car idles at a higher RPM that it used to, and it varies too. Attempting to lower the idle even just a little causes heavy shaking and stalling. (3) Turning on the AC causes a drop in RPM, shaking, and near stall. I remember the car would actually increase the RPMs slightly in the past. (4) About 2-3 minutes after turning on the car and driving no more than a mile, a noticeable restraint/power loss occurs. It's kind of hard to describe but its like someone just hitched a trailer. (5) Accelerating in 5th gear isn't what it was and causes vibration in the stick shift.
Any advice appreciated. Thank you.
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Pete B wrote:

Not that this will solve the problem, but this should point you in the right direction. It sounds like your car is stuck in 'cold' mode. When a car starts, temp sensors tell the engine computer that the car is cold, so the computer runs a cold mode program. This program basically has the car running very rich, so you get lousy mpgs in cold mode. As the car warms up, the computer eventually swithes to normal run mode, where it now relies on the O2 sensors to get the ideal feul and air mixture for optimum operation, and hnce better mpgs lower rpms and better power.
That 'hitching of a trailer' feeling follows this path. Your car is warm, but the computer still thinks it's cold, so it still runs the cold mode program, and running cold mode on a warm engine is even worse than cold on cold. Running rich really loses power now.
I think your feul injectors may never have been the problem and you need to find a new mechanic.
Hope this helps.
CD
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Thanx for your reply. Two of the injectors were leaking fuel onto the engine creating a smelly, not to mention a hazardous condition. So, they had to be replaced, no question. The same thing happened two years ago with two other injectors, so this time I told them to do the remaining four. One of them turned out to be faulty electrically, or so he said. By the time he figured that out, too much else was touched. I doubt that replacing the other parts, however unnecessary it was, is the cause. Something else was touched, changed, or manipulated in some way (OR) the faulty injector is still bad despite the mechanics efforts to fix it himself. When the weather is very cold, it spits out a ton of smoke, never hits the fast idle and is very rough (RPM about 1000-1100 constantly, hot cold you name it). You can floor the car and it barely moves. Today, it did the same thing, stalled out, and I've not been able to restart it at all, even after waiting in 1/2 hour intervals. Cranks, sounds very close to catching, but never does. Fortunately, I was at home when this happened. But, it looks like I'm screwed now. Winter is only starting.

from 20

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Pete B wrote:

Wrong/cheap injectors and not replacing all of them as a set. First, unless you used factory injectors and the originals are in perfect condition, you now have a mismatched set of injectors that very likely deliver different amounts of fuel to each cyl. Also many of the aftermarket injectors spray like a squirt gun rather than a fan spray pattern. Until these are replaced with a -set- of good/factory injectors, I doubt you'll ever get the car to run right. Sounds like he ran into a mixture distribution problem with the few "new" injectors and tried to cover it up by richening the fuel mixture.
Also what sort of tuneup parts were used? Bosch platinums can make these cars run like total crap as well. You need to find someone who knows about these old straight 6 datsuns, like a Z-car shop.
--
Steve

http://www.atlantaracing.com
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