MPG question
Thread Starter
Racer
Joined: Jun 2005
Posts: 363
Likes: 0
From: Jacksonville, Fl
MPG question
The other day I was going 85 in 6th gear and the mpg said I was using 45 mpg (accelerating, not cruising).
The engine is stock. If this is wrong, which I am pretty sure it is, how do I fix it?
The engine is stock. If this is wrong, which I am pretty sure it is, how do I fix it?
Burning Brakes
Joined: Sep 1999
Posts: 786
Likes: 31
Cruise-In II Veteran
There is a good chance that a fuel injector has gone bad.
If the 1985 alogrithm is similar to my 1990, the computer looks at the injector pulse time as a basis for computing Instant MPG. The shorter the injector is open the less fuel is being used = higher MPG.
If an injector has gone bad (open) and is dumping fuel into the engine, the Oxygen Sensor will detect a Rich condition and the computer will cut back on the injector pulse time in order to maintain the correct air/fuel ratio.
Unfortunatly, while this looks like great fuel mileage it is not a good condition.
Quick and dirty tests:
1. Pull the spark plugs. The one that is all black is in the cylinder that has the bad injector.
2. With an Ohm Meter, test the resistance across the windings of each injector. There should be at least 11 ohms resistance. any thing less indicates the windings have shorted out and the injector has failed.
3. Check the fuel pressure at the Schrader valve on the right rear of the intake manifold. If it doesn't hold pressure after turning the engine off, a fuel leakdow is occuring somewhere-most likely at an injector.
4. Use a scanner to find out. This will give LOTS of info on how the engine is running.
If the 1985 alogrithm is similar to my 1990, the computer looks at the injector pulse time as a basis for computing Instant MPG. The shorter the injector is open the less fuel is being used = higher MPG.
If an injector has gone bad (open) and is dumping fuel into the engine, the Oxygen Sensor will detect a Rich condition and the computer will cut back on the injector pulse time in order to maintain the correct air/fuel ratio.
Unfortunatly, while this looks like great fuel mileage it is not a good condition.
Quick and dirty tests:
1. Pull the spark plugs. The one that is all black is in the cylinder that has the bad injector.
2. With an Ohm Meter, test the resistance across the windings of each injector. There should be at least 11 ohms resistance. any thing less indicates the windings have shorted out and the injector has failed.
3. Check the fuel pressure at the Schrader valve on the right rear of the intake manifold. If it doesn't hold pressure after turning the engine off, a fuel leakdow is occuring somewhere-most likely at an injector.
4. Use a scanner to find out. This will give LOTS of info on how the engine is running.
Drifting
Joined: Jan 2005
Posts: 1,354
Likes: 325
From: San Antonio Tx
So if you install bigger injectors, at 60 mph you use the same amout of fuel but with a shorter pulse width. Will the instant mpg computer then be inaccurate or does changing the injector constant in the ECM correct for this? I'v been wondering about that.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Increasing injector size is similar to bumping fuel pressure; the ECM will come with higher than actual fuel economy values.
For example I'm running 48psi instead of the stock 44.5psi so fuel flow is actually 3.8% higher than the ECM sees. This means that the mpg reading is about 1mpg too high at a 31mpg.
I only check MPG on a flat highway with cruize set.
For example I'm running 48psi instead of the stock 44.5psi so fuel flow is actually 3.8% higher than the ECM sees. This means that the mpg reading is about 1mpg too high at a 31mpg.
I only check MPG on a flat highway with cruize set.
