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1979 Riviera Fuel System Problem


Jolly_John

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Hi, Gang. A friend of mine has a '79 Riviera with the Olds 350 engine. Stock 4-barrel carb, mechanical fuel pump, and no computers of any kind (no ECM, no carb or distributor computer...nothing). The car is equipped with the charcoal cannister (one vac. controlled valve in and out that vents the carb fuel bowl, plus two other cannister connectors for gas tank vapors).

The car runs great on the highway when the gas tank is full, and down through about two-thirds full. However, once the fuel gauge shows about two thirds full, the engine acts like it is running out of gas, when climbing a long hill at around 65 m.p.h. You can be climbing the hill at 65 m.p.h., and the engine will stumble if you step on the gas a little to maintain the 65 m.p.h. If you slow down to 55 m.p.h. and are easy on the gas pedal, the car will handle the hill.

There seems to be no fuel delivery problem in around town driving at lower speeds, even when the tank is less than two-thirds full. Again, the car runs great on the highway and will zoom up any hills when the tank is over two-thirds full. The only stumbling or "starved for gas" problem exists when trying to maintain higher speeds while climbing a hill on highway, ONLY when the gas tank is less than two-thirds full.

My friend is not on the computer, so I'm posting here to give him a hand. We will both appreciate any suggestions for I.D.ing the source of this problem you might have. Thanks. John

Edited by Jolly_John (see edit history)
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Do you think that a dealer is the best place to take a 31 year old car?

Unless you knew the mechanics there very well, I would be more inclined to find any reputable shop that had a 60+ mechanic who still owned and specialized in pre-computer, carburetor-equipped cars.

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Good discussion, guys.....but does anyone have ideas about what the problem might be caused by. If the fuel pump is failing under load (per Rivieraman), why would the car exhibit no problems, when the gas tank is two-thirds or more full?

Has anyone tackled a similar problem? Thanks. John

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Good discussion, guys.....but does anyone have ideas about what the problem might be caused by. If the fuel pump is failing under load (per Rivieraman), why would the car exhibit no problems, when the gas tank is two-thirds or more full?

Has anyone tackled a similar problem? Thanks. John

Many pumps in general will operate more efficiently when less suction lift is required. When the tank is full, the level in the tank is probably closer or perhaps even above the level of the fuel pump (depending on how level the car is sitting) inlet so the fuel will flow by gravity to the pump inlet creating a positive inlet pressure vs a negative one if it has to lift the fuel vertically to get to the pump inlet. So if the pump was very weak or close to failing, a full tank would help it along. I have to admit its rare for a pump to get weak. Usually they work or they don't but I suppose anything is possible.

Before the pump is replaced I would first check any rubber fuel hose on the suction side of the pump either at the pump and/or back by the tank. I'm not familiar with the 79s to know where the hoses are. It could be possible a weak or perhaps incorrect type of hose is collapsing under the negative suction pressure of the pump. A hose/line could also be bad/leaky and allowing the pump to suck air under certain conditions

Edited by JZRIV (see edit history)
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Sorry guys it is easy for me to say go to the Buick Dealer up here in good old Buick land. Flint Michigan i worked at a Buick dealer for 12 years so it is easy for me to know someone and find someone. But i have heard of alot of fuel pumps doing this kind of thing before thye fail. But also Jason brings a good point up so check that as well

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Here's another fuel pump related "thing" to consider . . .

That fuel pump is probably a "two-line" pump . . . one for fuel feed and one for excess fuel return.

Here's what happened in our shop several years ago. A man from western TX came in with a motor home. The performance complaint was "low power". This was mid-afternoon and he left home (about 200 miles away) at about 8am that morning. The thing would only run about 40mph. He'd been into several dealerships before he got to us . . . all of which found nothing wrong with the engine. A private shop installed an electric booster pump, which helped some, but didn't fix it completely.

Our innovative tech came in and asked for a carburetor jet (back when we still had a few in stock). I got him what we had and he selected the smallest one. I asked what he was doing. He told me the story and then left. I saw the motorhome drive out soon thereafter. Later, he brought me the RO to charge the jet out on. Then he told me the more complete story.

The man and his family had left home early that morning, only to find that the motorhome would now only run about 40mph on flat land. He made it to the nearest GM dealership to get it checked out. "No problem found", but there was a "problem". He drove to the next largest town on the Interstate (still on reasonably flat land). Same song, second verse. He continued eastward. He found a private shop and they added the electric pump at the back of the vehicle, as a booster pump. It helped some, but still not a complete fix. Now, he could drive 50mph on the flat land . . . which was better.

So, after hearing this story, our tech wanted the carb jet to restrict the return fuel line somewhat. I think he might have used some vice grips to restrict the line to see how it might react, so he then came after the jet to effect a better fix. With the carb jet restricting the fuel return line, that put more fuel to the float bowl . . . and the motorhome THEN would run 70mph on the primaries and with little throttle input. The tech said the owner was smiling ear to ear when they did that.

The booster pump helped get more fuel to the mechanical fuel pump and then to the carb, but because it was over-powering the return line. Restricting the return line further put more fuel to the carb, by default, although the return line was still working.

The fuel pump's return line has an orifice in it, otherwise it'd return too much fuel to the tank. Kind of made me wonder if lots of the hot fuel handling issues we had in the middle 1980s was really caused by "somebody" increasing the size of the orifice for the fuel pump's return line. Or perhaps something was making the orifice increase with time and use?

It's possible that the same thing might be happening with the "full tank" performance vs. "partial tank" performance. With a full tank, the fuel would have enough "push" (from the weight of the fuel and vertical positioning of the lines in the chassis) to put more fuel to the pump, but with less fuel, that "push" would be decreased.

All things considered . . . you might get a new OEM-spec pump (some of the ACDelco pumps are VERY reasonably priced these days, but I don't know about that particular one) and see what happens. If the situation persists, then you might scrounge an old carb jet and put it in the return fuel line (at the pump)--it should push into the fuel line, hopefully, before you reconnect it to the pump. Or a "thick wall" fuel line of the next outside diameter "down" from the existing return line--"thick wall" meaning that the inside diameter is smaller than normal for the same outside diameter. Or perhaps a small cup plug with a smaller hole drilled into it, so the return line system would still work, placed inside of the return line at the pump?

Just some thoughts . . .

NTX5467

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Willis and Guys,

This was a very common failure on 70's to 80's Oldsmobile V-8 engines. As the pump would age, it would loose the ability to supply the carb AND the return line with adequate fuel. We would test it the same way, vise grip off the return and drive the car again. If it ran better, we came back and INSTALLED A NEW FUEL PUMP!! Restricting the return line is only a band-aid and the pump will fail TOTALLY soon after the restriction is installed. This is only a "limp home" repair at best!

I think Jason is right on about the fuel tank level and the vacuum needed to draw the fuel from the tank when full compared to the level dropping.

You can tell if an Olds engine fuel pump from 79 has been replaced or not, the original usually had engine color over spray on it. If your pump has any engine color paint on it at all, replace it ASAP!

Tim

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Wow, Guys....you are super. I especially thank Willis and Tim this time around. Yes, the fuel pump on this engine is branded "A-C" and it has blue engine paint on it. Sounds like an original to me. I'll try the pinch off the return line procedure first, when the car is acting up, and see what happens. We may have it, Gang! I'll be sure to post an update. Best, John

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Thanks for the additional information, Tim.

As I mentioned, those pumps are usually pretty inexpensive (at least the ones I've bought have been, ACDelcos or otherwise). Not worth the agony AND expense of being on the side of the road--to me. As it has the "production overspray" on it, I'd go ahead and replace it for good measure, insurance, and peace of mind.

Besides, if you live in a area with ethanol'd fuels, that earlier pump's diaphram is not really rated to handle such fuels.

Just some thoughts,

NTX5467

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