Fuel/Air ratio question

[CharlesBrown]

My '88 vett has 27400 miles on the clock. As you might expect, it just sat for quite some time in someones garage. I've replaced a number of things however plugs & spark plug wires were yet original. Did not seem that important since the car started easily and ran well (I guess). I wanted to find out what kind of power the car could produce so I went to the local speed shop for a dyno test. The test was halted because the preliminary run up showed a very high fuel/air ratio i.e., 16.3:1. The owners of the shop said the full dyno test could seriously impact the engine. Their suggestion was to replace plugs & wires. Their price was reasonable but just to be on the safe side, I asked by mechanic (local garage guy) what he would charge for the new plugs & wires. After hearing the above he said, "What's plugs & wires have to do with a lean fuel/air ratio." If the dyno people are right I've no problem replacing the plugs & wires. but I don't want go on a "replace the part." scavenger hunt. So here's the question, with the above info. does this sound like a plug & wire problem?

 

[JAKE]

If the engine wasn't missing, I wouldn't first suspect plugs or wires.

Instead, I'd suspect fuel (is yours fresh?); fuel pump output; fuel pump sock, fuel pressure at the regulator; fuel pressure regulator, fuel filter partially plugged, injector(s) clogged or partially clogged; and even ignition timing.

Engine's obviously going lean under power, as opposed to normal part throttle operation where it ran well. So I'd suspect you have a fuel delivery issue.

Just my thoughts.

Jake

West Point ROCKS!

 

[vetteboy86]

At what point was the 16.3 recorded? RPM and Load Variable. I would check fuel and spark.

 

[CharlesBrown]

On the dyno @ 3000 rpm.

 

[LLC5]

My '88 vett has 27400 miles on the clock. As you might expect, it just sat for quite some time in someones garage. I've replaced a number of things however plugs & spark plug wires were yet original. Did not seem that important since the car started easily and ran well (I guess). I wanted to find out what kind of power the car could produce so I went to the local speed shop for a dyno test. The test was halted because the preliminary run up showed a very high fuel/air ratio i.e., 16.3:1. The owners of the shop said the full dyno test could seriously impact the engine. Their suggestion was to replace plugs & wires. Their price was reasonable but just to be on the safe side, I asked by mechanic (local garage guy) what he would charge for the new plugs & wires. After hearing the above he said, "What's plugs & wires have to do with a lean fuel/air ratio." If the dyno people are right I've no problem replacing the plugs & wires. but I don't want go on a "replace the part." scavenger hunt. So here's the question, with the above info. does this sound like a plug & wire problem?

A misfire can give you a lean condition, but if there is no misfire then you need to look elsewhere. If your "speed shop" knows what they are doing, then there should be no guessing as to which way to proceed, you may want to find another dyno tuner.

 

[minifridge1138]

16.3:1 is lean.

If you had a fouled plug or bad wire, then I would think you'd run rich because you have unburned fuel.

I think optimal power occurs at approximately 12:1 and optimal engine temperature occurs at 14:1.

I'd bet you have 27,400 miles on the original fuel filter.
I would start with that, just because 21 years is a beyond geriatric for a fuel filter.

Then I'd run a bottle of techron to clean out the fuel injectors.

After that I would measure the fuel pressure.
If it is to spec, then you have enough fuel getting to the engine. The question is why isn't it making it into the combustion chamber.
That could be a dirty Mass Air Sensor or Mass Air Pressure Sensor. That would make the car think there was less air than there really is, so it would use less fuel than it should.

Is the check engine light on?

 

[LLC5]

16.3:1 is lean.

If you had a fouled plug or bad wire, then I would think you'd run rich because you have unburned fuel.

Yes, you will have unburned fuel, but you will have a lot more oxygen content than fuel. Depending on the reason for the misfire, you could have a lean condition.

 

[Hib Halverson]

If the chassis dyno test progressed to the point that the engine was loaded enough that it went into open loop and what should have been power enrichment then 16.3 is way, way lean.

At WOT, serious detonation, perhaps more than spark retard would be able to fix, would occur

The problem I'm having, here, is if the engine is really that lean at WOT, the check engine light should have come on.

Is there some way you can post the actual dyno data showing both RPM, rear wheel power and the measured air-fuel ratio? I'd like to see it before I offer further suggestions.

 

[minifridge1138]

Yes, you will have unburned fuel, but you will have a lot more oxygen content than fuel. Depending on the reason for the misfire, you could have a lean condition.

That makes perfect sense.

 

[CharlesBrown]

Check engine light is not on...and the fuel filter was replaced last year. doubt if there is more than 1000 miles on it.

16.3:1 is lean.

If you had a fouled plug or bad wire, then I would think you'd run rich because you have unburned fuel.

I think optimal power occurs at approximately 12:1 and optimal engine temperature occurs at 14:1.

I'd bet you have 27,400 miles on the original fuel filter.
I would start with that, just because 21 years is a beyond geriatric for a fuel filter.

Then I'd run a bottle of techron to clean out the fuel injectors.

After that I would measure the fuel pressure.
If it is to spec, then you have enough fuel getting to the engine. The question is why isn't it making it into the combustion chamber.
That could be a dirty Mass Air Sensor or Mass Air Pressure Sensor. That would make the car think there was less air than there really is, so it would use less fuel than it should.

Is the check engine light on?

 

[CharlesBrown]

No data was produced from the 3000 rpm run up, just the bill!

If the chassis dyno test progressed to the point that the engine was loaded enough that it went into open loop and what should have been power enrichment then 16.3 is way, way lean.

At WOT, serious detonation, perhaps more than spark retard would be able to fix, would occur

The problem I'm having, here, is if the engine is really that lean at WOT, the check engine light should have come on.

Is there some way you can post the actual dyno data showing both RPM, rear wheel power and the measured air-fuel ratio? I'd like to see it before I offer further suggestions.

 

[vetteboy86]

What was the coolant temp of the car at the time of the test? I'm wondering if it was in open or closed loop, somewhat on the lines of Hib's post.