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post #1 of (permalink) Old 11-25-2002, 02:56 AM
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MPG going very low!

This summer I had good MPG (around 18) in the city. It was even better (about 22) when I did a longer trip (open road), and around 20 with mixed road (city/highway).
Now, the winter's near, temperatures are low (some 50 and going lower), and my MPG has gone to a half! I'm doing around 10 MPG! (it's a 2.5 Ltr) [img]images/graemlins/mad.gif[/img]
I drive in the same way I did before, I use the same fuel. Only thing I changed since was that I rerouted the air intake, which was originally between the radiator & the front grille (stock '93 YJ), so now the tube goes from the airbox (Stock), makes a 180-degrees turn UP and is aligned with driver's side grille-reinforcement-bar (the one that goes on top of everything, from the grille to the firewall). The air it gets is now a bit hotter than before, I presume (and the tube points backwards), but I didn't notice any lack of power or any other change in engine behaviour.
Could that air-intake be the reason? Could it have so much importance to MPG?

If it helps a bit, in the city I change gears a bit over 2000 RPM (some 2200-2500), so when I put in the next gear it's low RPMs, but not stalling. The minimum.

BTW, how do you guys experience the low temperatures & MPG ratio? Going low, or going VERY low?
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post #2 of (permalink) Old 11-25-2002, 03:06 AM
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Re: MPG going very low!

Unplug the coolant temperature sensor for the computer. If your milage comes back up, replace the sensor.

Other than that, watch for a neighbor with a siphon hose.
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post #3 of (permalink) Old 11-25-2002, 03:12 AM
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Re: MPG going very low!

</font><blockquote><font class="small">In reply to:</font><hr />
the coolant temperature sensor for the computer

[/ QUOTE ]
Where is that sensor, how to locate it? What does it do, and should I expect any changes while running without it?
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post #4 of (permalink) Old 11-25-2002, 03:28 AM
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Re: MPG going very low!

I don't know where it is and I'm just assuming it is computer controlled being a '93. When you unplug it you will get a check engine light but the computer should compensate for it. If it's giving a low temp reading, the computer thinks the engine is cold and runs it rich like a choke.

I had it happen on my SBC TPI, nobody could find the problem. I bought an Auto-Xray and the first thing I noticed was the computer thought the engine temp was -38 degrees. I knew that couldn't be because ambient was 65 that day. Unfortunately, -38 is two degrees above the fault threshold and the computer believed it. Replaced the sensor and my problems went away.

Do you have any other associated problems like poor throttle response but improves at WOT (wide open throttle)?

Bet you were surprised to get an answer this time of night (or day in your case).
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post #5 of (permalink) Old 11-25-2002, 03:35 AM
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Re: MPG going very low!

Yep, sorprised!
Here's 11:32 AM, but over there it's dead night, I presume... some 2-3 AM, no?
I found some info on testing that sensor with multimeter and a table of resistance it should read regarding the temp. I will check that today &amp; see...
post #6 of (permalink) Old 11-27-2002, 07:44 AM
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Re: MPG going very low!

I checked the resistance. It's surposed to be "under 1350 Ohms on a 93+ YJ, with the engine at operating temp". Mine was 1018 Ohms. So it's OK, I surpose. In the morning I checked it again, and it was also in the limits given. So I guess that's not it.
Any other ideas?
post #7 of (permalink) Old 11-27-2002, 07:49 AM
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Re: MPG going very low!

</font><blockquote><font class="small">In reply to:</font><hr />
Unplug the coolant temperature sensor for the computer. If your milage comes back up, replace the sensor.

[/ QUOTE ]
I checked it with Ohmmeter, and it seems OK.

</font><blockquote><font class="small">In reply to:</font><hr />
watch for a neighbor with a siphon hose

[/ QUOTE ]
Nope. I have a locking tap on the tank, so one has to break it to get in.

Any other ideas?
post #8 of (permalink) Old 11-27-2002, 09:30 AM
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Re: MPG going very low!

Well, the temp sensor is a common problem and easy to check.

Next is to do a fuel pressure leak down test to see if you have a leaky injector. You need a fuel pressure gauge, run the engine and check for pressure, make sure the pressure is not too high. Then shut the engine off and see how much pressure you loose in about an hour or so. Check at 15 min. intervals. You might try some of the injector cleaner additive in the tank.

If you are figuring on keeping it for a while, see if there is a scanner available for it like Auto-Xray. They are a big help in real time monitor mode, tells you all the sensor readings and what the computer is sending for an injector pulse width.
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post #9 of (permalink) Old 11-29-2002, 12:29 AM
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Re: MPG going very low!

</font><blockquote><font class="small">In reply to:</font><hr />
fuel pressure leak down test

[/ QUOTE ]
Actually there is a permanent fuel-gauge mounted on the end of that piece of fuel-pipe attached to the engine head. It is there permanently, was thwere when I bought the YJ. I'll check it tomorrow morning. Just for the info, what pressure should be there?

</font><blockquote><font class="small">In reply to:</font><hr />
Auto-Xray

[/ QUOTE ]
??? where should I look for that piece of equipment? This is the first time I ever heard about that... How much does it cost, how it looks like, where does it connect...?
post #10 of (permalink) Old 11-29-2002, 07:34 AM
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Re: MPG going very low!

Not sure what your pressure should be, you'll need to get the specs, I'd guess somewhere around 35 lb. If it's too high it's a problem and the regulator has gone bad. What I suspected though was a leaky injector, they get dirty and don't close completely. To check for that, you get the system up to pressure and then, with the engine not running, check for pressure loss over time. If it looses very much pressure in say 15 minutes, it could be leaking out through the injector.

The Auto-Xray is a scanner that will give real-time readings of all the sensors and output signals to and from the computer. Here is a link, the one you need is all the way at be bottom of the page. The price shown is $200 but I bought mine cheaper so shop around. I'm not pushing Auto-Xray, There are other brands and you may be able to find software for a laptop. I have the GM SBC TPI in mine and there is plenty of support, even some freeware support on the net. I wouldn't expect as much support for yours, but you might surf the net.

You also need to verify you're going into closed loop. If you have a bad O2 sensor, you won't go closed loop, but open loop should still give you decent mileage. I did assume that this was a sudden change, that you did the normal checks for bad plugs and that you have no service engine light on.
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