HELP: Loss of power and MPG. - Defender Source
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  #1  
Old February 20th, 2014, 02:50 AM
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Alex Lapidus
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HELP: Loss of power and MPG.

I have a 1994 ST D90. At about 100K the original 3.9 V8 engine threw a rod and was replaced with a a new 4.6L by a local shop here in Portland, OR. Initially it was awesome. Tons of smooth power at all RPM's. I've since put about 20K on the engince and about 8-10K ago it felt like it lost power, specifically in the mid-range of the RPM's. The power would build from the bottom end, then flatten out, then pick up again at the higher end of the revs. Additionally these symptoms seem worst when the enginge is cold and for the first maybe 5 minutes of neighborhood driving. Also the MPG's dropped from 16-17 on a good day to maybe 10 now. It might be my imagination but when I'm engine braking it doesn't backfire, but sort of burbles a little. It still hums along at 65 MPH, but I have to down shift up hills that I used to charge up pulling a motorcycle trailer in 5th gear. At one point I noticed that the bolts holding the vacuum advance on the distributor had completely fallen off and the vacuum advance was loose. I tried to find the right size/pitch bolt, but couldn't and finally zip-tied it back on in a somewhat half-assed, but relatively tight way. I became convinced that the issue was my oxygen sensors and just replaced them a few days ago without any change in my symptoms. I pulled off the kickpanel to check the codes, but the screen isn't showing anything.

1) Is it likely my vacuum advance?
2) If the On Board Diagnostic box shows nothing, does that mean there are no codes to be read?
3) What else could it be?
4) What's my next move?

I would REALLY appreciated anyone's thoughts.

Thanks, Alex
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  #2  
Old February 20th, 2014, 04:06 AM
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ed angel
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Get the vacuum advance fixed first . The obvious needs to be eliminated first . The next thing I would check is fuel pressure or just change the filter if it hasn't been changed since the swap . I have had similar conditions with a bad Mass air flow sensor that didn't set a code . You might take it back to who ever did the motor swap , they will probably be able to help you out .

Ed
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  #3  
Old February 20th, 2014, 06:40 AM
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Bill Adams
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Eliminate all possible manifold vacuum leaks, especially the faulty distributor. If the vacuum advance is hanging loose, it is definitely not doing its job of advancing the ignition. It would explain your poor performance.
Oxygen sensors do nothing more than adjust lean-rich after the engine has warmed up. Even with them disconnected the ECU can use the MAS to figure out bandwidth.
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  #4  
Old February 20th, 2014, 08:04 AM
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Jason England
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Sounds exactly like vacuum advance not working!!

Get it fixed and replace the vacuum hose. Also make sure the connection into the plenum is clear.
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Old February 20th, 2014, 08:11 AM
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Joshua
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Is the vacuum nipple on the plenum just a simple air pass through that you can poke something sharp into to clear any blockage?
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Old February 20th, 2014, 09:12 AM
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Robert F
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pull the fuse to the ECM to clear its ram then see if your power comes back. If it does, you have an exhaust or engine leak and it is over fueling to compensate.

Check timing and that there is no corrosion in the distributor cap. These engines seem to be way more sensitive to faulty ignition components than other makes

I'd also pull the top manifold to make sure the trumpets are all where they are supposed to be and not loose.

If all that checks out, do a compression check to make sure you are working with a good motor.
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Old February 20th, 2014, 10:57 AM
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James Wallace
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Another lurking culprit, besides the obvious vacuum advance, would be to check the coolant temperature sensor. This is an often overlooked item which does fail and cause a bunch of power and fuel consumption related issues. If it is defective, it will keep the engine in warmup mode, running rich. This manifests itself with low MPG and is a bit on the smelly side. They are a resistance unit and can be checked with a VOM meter. You can get units from O'Reiley's for a lot less (under $20) than the usual suspects, they are common to a lot of cars.

A brief explanation:

http://www.howstuffinmycarworks.com/...mp_sensor.html
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Old February 20th, 2014, 11:25 AM
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Jason England
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Quote:
Originally Posted by Dogpilot View Post
Another lurking culprit, besides the obvious vacuum advance, would be to check the coolant temperature sensor. This is an often overlooked item which does fail and cause a bunch of power and fuel consumption related issues. If it is defective, it will keep the engine in warmup mode, running rich. This manifests itself with low MPG and is a bit on the smelly side. They are a resistance unit and can be checked with a VOM meter. You can get units from O'Reiley's for a lot less (under $20) than the usual suspects, they are common to a lot of cars.

A brief explanation:

http://www.howstuffinmycarworks.com/...mp_sensor.html
very good point ... this is not the same sensor as the one for the temp gauge. so your temp could read fine and this fault was still present.

Given the vacuum advance is attached by cable tie ... i'd be looking in that area.

Also if the truck has recently been 'tuned' and someone set the timing with the vacuum hose still attached it would cause these symptoms too.
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