Horrible Gas Mileage Problem

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ash10hunt
Posts: 40
Joined: Thu Mar 11, 2021 6:29 pm
Car: 1999 Infiniti Q45

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First post on here!
I got my 1999 Q45 in November with a little over 200,000 miles on it. Other than some rust that I planned to fix it seemed to be in perfect shape. I drove it home about 300 miles on just over a tank of gas. After having it for a few weeks the p1105 engine code came on so I replaced the MAP sensor. I was still getting bad gas mileage, so I replaced the spark plugs, which also did nothing. I tried resetting the ECU, which cleared the code for a few weeks, but the gas mileage was still bad, I'm talking like 5-8 miles a gallon. I cannot find any gas leak from the tank, lines, or in the engine bay. And I tried the starter fluid trick to try and find vacuum lines, didn't find any, although I know that's not the most foolproof technique.
I'm thinking it's just a wiring issue with the MAP sensor, so I'm planning on scouring the engine bay for bad wires and testing the MAP voltage.
Any tips for making this easier or ideas as to what the problem could be would be great, thanks!


ash10hunt
Posts: 40
Joined: Thu Mar 11, 2021 6:29 pm
Car: 1999 Infiniti Q45

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Update: I just drove like a mile and heard some sort of clanking sound from the front when I went over bumps, I don't think that its related. It also had really rough idle, and when I turned it off and then started it again, the rpms when down to like 500, almost died, and then finally got up to about 800.

intermech
Posts: 12
Joined: Mon Mar 08, 2021 10:52 am
Car: '03 Infiniti Q45

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I am new here but my 2 weeks old 03 got about 16mpg. Have you got it figure it out yet?

BadQ45t
Posts: 3249
Joined: Fri Jul 26, 2002 7:54 am
Car: 21' Leaf SL Plus (wife's car)
05' X-Terra S 4x4
97' Q45t SOLD! Miss the old girl
84 300ZX (Original Owner)
2016 Mercedes Benz S550e V8 Power and 35 MPG
Location: Half Moon Bay, CA

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Wow, that is bad, I got around 16-22 in my 97' before I sold it but drove it for 19 years. The clucking could be a few things, first has the car has new struts and shocks installed? If not then I bet all 4 are leaking bad and that is an issue as replacement parts are a pain and expensive. Second check the tension rods, they have rubber bushings that go bad. Gas mileage seems odd and you may have to get a mechanic to take a peak at that to see what is going wrong. Does the car run strong and rev up freely. These 4.1 engines got a bad rap for not being a 4.5 but they were whisper quiet and smooth as silk.

Ryantzer
Posts: 205
Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
Location: Phoenix, AZ

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Connecting an OBD2 scanner will enable you to see the engine data in real time, specifically the fuel trims, to see if something's not right.

3Q Jay
Posts: 2545
Joined: Sat May 08, 2004 6:23 pm
Car: 94 Q45a
95 Q45a (sold)
97 Q45t (sold)
01 B15 Sentra (Daughter's)
Also Mine...
2010 A6 Avant
1977 F-150 (460!)
Location: Florida Coast

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ash10hunt wrote:
Fri Mar 12, 2021 8:36 pm
Update: I just drove like a mile and heard some sort of clanking sound from the front when I went over bumps, I don't think that its related.
I don't think that's related to gas mileage.
But.
On my FGY33, the upper strut mount (shock tower) separated and made a noise like that that took me a while to figure out.
you can verify by (removing the rubber bolt cap if necessary), and sticking your finger in the top hat hole while doing a lil bounce on that corner. If it's separated, you'll know.


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