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Quadrajet Conundrum [message #271272] Thu, 05 February 2015 23:27 Go to previous message
Vince Kirkhuff is currently offline  Vince Kirkhuff   United States
Messages: 12
Registered: May 2009
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Junior Member
First, let me apologize for the length of this post... if you don't wish to read my diatribe, I understand; I won't be offended if you choose to simply move on.

My name is Vince Kirkhuff; I have met some of you at some of the Western States Rallies. I've posted here only a handful of times, because I can usually find the answer to any GMC situation by searching the Forum archives. I have not had any luck finding a similar situation to what I am now experiencing.

I live in San Luis Obispo on the Central Coast of California. I drive a 1977 Eleganza II with the 455 and a stock Quadrajet. I've owned it 6-1/2 years bought it from the original owner with 45,000 miles on it. It has about 56,000 on it now.

5 or so years ago I was experiencing classic vapor lock problems: Hot day, at altitude (3,500+ ft.), and starting off from an idle, the engine would stumble and backfire through the carb or die altogether. In an attempt to correct his problem, I installed a Carter P4070 electric fuel pump and inline filter just downstream from the tank selector valve. I used the selector valve as the power source, so the pump only ran when the auxiliary tank was selected. The pump was on a by-pass loop, with a check valve in the main line to keep all the gas moving toward the engine. The mechanical fuel
pump was in its original configuration.

This worked pretty well until one hot day I was trying to climb Greeley Hill Rd. out of Greely Hill, California, which is a fairly long, steep grade, and the fuel gauge was showing about ¼ tank. I was running the electric pump, due to vapor lock conditions, and heading up that hill, all of the gas sloshed to the rear (main) tank. My auxiliary tank was now dry, so I had to switch back to the main tank turning off the pump! The engine would not run at faster than an idle without the pump, but it wouldn't run at all on the (dry) auxiliary tank. I had to back down this narrow, twisting, two-lane country road about a quarter of a mile to where I could turn around. Luckily, I was able to nurse it the ½ mile or so back into town where there was a gas station.

So, my next modification was to hook up the electric pump to run all the time. I was happy to get rid of the bypass circuit with the T's, the check valve, and about 10 hose clamps, I figure my potential for leaks was drastically reduced. I was now running both the electric and mechanical pumps full time. It was with this set up that the Mysterious Lean Condition first manifested.

The first time I felt it do this, I was heading up Highway 5 me, the wife, the mother-in-law, and 3 dogs; on our way to a family camp out. About 2 hours into our trip and this was nighttime it starts to feel like it's running out of gas; but I've still got at least ½ a tank, according to the gauge. But I think I'm feeling it hesitate... then falter, then finally backfire through the carb... this takes about 10 minutes, and 10 miles driving down the road. The more gas I give it, the worse it runs opening the throttle is just leaning it out. The engine finally dies and we coast to a stop on the shoulder of the freeway.

So we sit there a minute, and I'm trying to think of what to do... diagnose my fuel problem? Am I going to repair it? On the side of Hwy 5? In the dark? Call a tow truck? I'm thinking, if we weren't on the shoulder of the freeway, we could just go to bed and worry about it in the morning. So then I'm thinking, if I can get it running, at least I can maybe get it off the freeway. So I crank the engine, and it fires right up, and it runs fine, and we accelerate into traffic just like nothing... it's getting plenty of gas; the secondaries are clearly working, and it runs great! In 10 minutes, it does it again. Wait a couple of minutes, start it up, and off we go again. I finally figured out I could keep it running if I went a lot slower like under 55 m.p.h. It seemed like it had enough fuel to maintain that speed, but no faster. We found a campground for the night.

In the morning, I called Kanomata's shop and talked to Nick, and he said yeah, it sounds like a lean fuel issue, and suggested a fuel filter. So I bought one and replaced it when we got to our destination. We were there for 2 nights, and then headed home. Out on the road it ran fine, so I thought I had fixed it. Then, a couple of hours into the drive, it started doing it again. We had a slow drive home that day, just taking it easy in the slow lane. That was a year and a half ago.

Since then, we've only had it out on long enough drives to create the problem twice. (A lot of our trips are within an hour or so of home, and it always runs fine for the first 90 minutes, at least.) I thought maybe something weird was happening between the 2 fuel pumps, so I have bypassed the mechanical pump. I also removed the fuel filter at the carb, and replaced the ignition module for good measure. It still does it after those changes. It will do it with a nearly full tank of gas, a part tank, or nearly empty main tank or auxiliary. The last time it did it, I got a big clue: when it started to feel like it was running out of gas, I pumped the gas pedal like mad, and it ran better. So this makes me think the carb is getting fuel there's obviously fuel in the float bowl, but for some reason it's running lean, and the accelerator pump enriches it enough so it runs okay. I was able to nurse it along for about 10 miles down the freeway by pumping it to help it up little hills and such; but still, it eventually died. Again: Wait 1 or 2 minutes, and it starts and runs fine.

Before I took it out on that last road trip, I searched the Forum archives for similar problems, but didn't find one exactly like mine... in that my problem fixes itself with a few minutes' rest. Thinking it may be electrical, I contacted Dick Patterson, who thought it sounded like a fuel problem and promptly talked me out of buying a distributor from him. It was his suggestion to remove the fuel filter at the carb, and check all the primary coil connections as well as the ignition module connections. I also installed a temporary pilot light to tell me if the fuel pump has power (it does). The tanks were dropped 4 years ago (very clean at that time) and new rubber lines were installed.

I read one story on the interweb of some guy with a Camaro or a Chevelle with a similar sounding problem, and he found it to be a problem with the power piston in the Q-jet something about a little plastic clip that holds the top of it, and it had broken and was getting in there sideways but I don't know enough about carburetors to know if that experience is applicable to my situation. I'm hoping that one of you all has maybe run into this same situation and knows just the thing to fix it.

Alternatively, of course, is that many of you are knowledgeable about all things Rochester (or all things GMC), and even if you haven't seen this manifestation, perhaps these symptoms are pointing to some particular part? Any help is appreciated!

Vince Kirkhuff 1977 Eleganza II San Luis Obispo, CA

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