3tas4me

I have been having a problem with my '84 T/A L69 5-speed car for a while now. Under hard throttle, it seems to shut down completely at the top of third gear (around 5K RPM). It acts as if it has run out of fuel. If I let off the accelerator and then get back on, it is running again. If I rev it out, but do it more slowly, it doesn't have this problem. I have been reading about the vapor lock problem that was common on these cars. I have owned the car since 1987 and had all the GM campaigns done at that time, including the electric pump in the tank. I took the car out today after starting it up cold and ran it after a short warm-up. The problem was not as bad, but was still there even with the engine not fully warmed up. Are there any solutions to the vapor lock problem that I could try other than the GM campaigns that were already done years ago? Would insulating the fuel line help? Could this be a different problem? The car has 62k miles on it.

sofakingdom

iTrader: (1)

See my signature for a hint.

Instead of making up "space alien" type "explanations" for what you see going on, try sticking with the SIMPLE.

If it acts like it's running out of fuel, it's because it's .... running out of fuel. No need to invoke "vapor lock" for that.

Try a fuel filter. It's in the fuel inlet nut, right behind where the fuel line goes into the carb. Be REAL CAREFUL with it; use a 5/8" line wrench on the fuel line and a 1" open end to hold the inlet nut still while you break the fuel line loose. DO NOT let the inlet nut turn until AFTER the fuel line nut is broken loose; otherwise your next post will be about "I trashed my fuel line now what do I do". And we sure don't want to see you put yourself needlessly through a bunch of THAT.

Speaking strictly as the owner of a 83 L69 car for the last 30 yrs or so.

3tas4me

Fuel filter should be good, probably has less than 2000 miles on it as I changed it recently. Since you own an L69 car, have you experienced this problem at all? It is a frustrating problem as it runs really good otherwise. I should also note that the engine is basically stock, no mods of any significance (K&N air filter, Z28 dual snorkel base and Magnaflow muffler).

3tas4me

Just as a side note, I do have many years of auto repair background if you need to supply more technical info.

sofakingdom

iTrader: (1)

I've experienced it in a number of cars; never that particular one. One that was particularly bad was a 454 Caprice; it could move enough fuel to drive normally, but towing a boat, it would do that BAD trying to drive up a hill. But the engine RPO code doesn't a whole lot matter, the parts are all basically the same in all the old carb cars, from the 50s on up to the intro of EFI.

It was ALWAYS a fuel delivery problem of some sort. What happens is, when the fuel demand is low, the fuel system can keep the carb full; but when the engine demands more fuel, it drains the bowl eventually, and starves. Then after a short time of little demand again (idling) the fuel system catches back up and it runs OK again.

Might want to consider trying the simple trick of removing the little check valve thing that's built into the fuel filter; just pull out the rubber insert thing and the spring and ball and all that, put it back, and see if it's better.

Might also be as simple as, the fuel pump is bad (broken spring), such that it can't pump enough fuel to keep up. Or, the "sock" filter in the tank may be plugged up, or the pickup line itself full of crusties. The problems in the tank are usually the result of sitting around for long periods of time and the gas in the tank turning into tar as it inevitably does. Or even, the upper bolt in the holes on the front of the motor, is too long, and is hanging up the fuel pump rod, preventing the pump from making a full stroke.



Make sure there's a bolt in that hole, with a drop of sealer on it; but also make sure it's AS SHORT AS POSSIBLE and still able to do its job. If you look into that hole you can see the pump rod.

"Vapor lock" WAS at one time a real hassle; it was caused by the gasoline in the main fuel line, which is under suction, flashing into vapor. Gasoline has been HIGHLY resistant to that since about the early 60s, by design. It basically NO LONGER EXISTS. Additionally, fuel systems such as these cars have, with a regulator built into the pump and a return line going back to the tank to constantly circulate cool fuel through the line, make it all but impossible even if your gas was a 50s or older formulation.

naf

iTrader: (7)

Your L69 has the in-tank pump. In addition to the above (and I've found the little check valves in the filters to be a problem before, myself) ensure your in-tank pump is operating. You can remove the fuel feed line from the mech pump and jump the relay to check. Probably third likely on the list after failing mech pump and filter, but still up there.

3tas4me

The bolts are all the original bolts installed by GM, so I would doubt that to be the problem. I can try the fuel filter trick and see what happens. There is no doubt the engine is starving for fuel under heavy demand, the question is "why". The fuel pump on the car is not the original anymore, but is probably 26 years old now. It was replaced with the Delco replacement for the fuel vapor lock problem. I realize what you are saying about today's gas blends, but when I park the car for short time after it is hot and then get back in it, it runs terrible for a short time. There is no doubt that the fuel is boiling either in the line or carburetor at that time. GM knew this was a problem, thus the multiple service bulletins to try and fix it. I can buy a new fuel pump and replace the one I have, but I hate just throwing parts at something until it is repaired. I appreciate the suggestions, I will just have to do some testing to narrow it down.

3tas4me

Oddly enough, a few years ago the car shut off on my wife while she was driving. We could get it to restart, but not stay running. Since I was out of state with it and had no tools with me, it was towed to a shop. They said they replaced the fuel pump in the tank (they gave me the old one) but the car still did not run any better. I trailered it home and had it repaired in 5 minutes. The carb base gasket had broken and created a huge vacuum leak. Makes me wonder now if they actually did install a new electric pump in the tank at all....

naf

iTrader: (7)

Another possible symptom of a weak fuel delivery system, especially with stock exhaust and the in-tank pump.

The in-tank should flow an easily detectable amount but you may not get a very high pressure reading off of it, if you were so inclined. A good mech pump replacement is the Carter 6626 at around $15-20, IF you need one.

five7kid

iTrader: (14)

I'd put the order of probability as:
1) Failed in-tank electric fuel pump
2) Cheap/clogged/otherwise problematic carb inlet fuel filter
3) Weak mechanical pump

If it's #3, I second the Carter M6626 recommendation. It's a direct replacement bolt-in, with higher capacity than the Delco units.

drjewalton

What about "valve float"? This term has been suggested to me as a common 305 problem, although I don't really know what it is.

My 84 L69 T5 does similar things when winding out the gears...get to around 4500-5000 RPM and it acts like it's starved for fuel. Lay off a bit and it recovers. Seems to also be harder to start after reaching operating temp than when you start it cold.

Driving under normal conditions, the car is flawless...just when its pushed to the 4500-5000 mark at WOT does it do this.

Also, I don't think mine has an in-tank electric pump...just mechanical on the block. Are all L69's supposed to have one? I can confirm mine is all original with 36k miles.

Fuel filter and mechanical pump are new.

Thanks

gt4373

Also, I don't think mine has an in-tank electric pump...just mechanical on the block. Are all L69's supposed to have one? I can confirm mine is all original with 36k miles.

Fuel filter and mechanical pump are new.

Thanks

Not According To My Shop Manual.

naf

iTrader: (7)

It is possible that some L69s do not have the electric in-tank pump. Look for the fuel pump relay near the brake booster. The oil pressure switch is usually tee'd off near the oil pressure sending unit behind/below the distributor (the switch is a two wire sensor).

Some vehicles were retrofitted by the dealership under service complaints though so I would suspect there may be some variation in actual equipment installed and placement.

3tas4me

Your symptoms sound exactly like mine. My car was retrofitted with the in tank electric pump as a dealer campaign. I suspect that when my car went to the shop a few years ago in the story I described above, they did not actually put a new pump in the tank. I have my old one because I asked for it back as proof they actually took it out. I don't hear a pump run when I turn the key on and am listening at the fuel filler. My car had several campaigns done for this vapor lock problem right after I bought it in 1987. I think they actually did 4 campaigns, with the electric in tank pump being the final one that seemed to cure the problem. I have checked with GM and that pump is no longer available. Not sure if the aftermarket would have it.

dlinger

iTrader: (4)

Had similar issues with our camaro. It had new fuel filter, rebuilt carb, new block fuel pump, new plugs, wires, coil. It would go away as soon as I replaced the fuel filter. Finally cut one open and it was full of rust.



Had to replace tank. Ours had the in tank pump also (not working and pickup broken).

Fuel filter is cheap and easy to replace (the first thing a mechanic will replace during troubleshooting even if told it is good).

gt4373

Damn I Never Saw A Fuel Filter Clogged That Bad.

dlinger

iTrader: (4)

And I just replaced the month before, probably less than 200 miles on it.

drjewalton

Even though my mechanical fuel pump (AirTex) was only a couple years old, I decided for $20 I'd again replace this part. I used a Carter pump this time. No more problem so far!

Ozz1967

iTrader: (4)

My car had the in-tank pump added at the dealer, I Think. I have no relay for it nor was there a fuse for it in the fuse block. But, it was in the tank, dead as a door nail. The mechanical pump pulled strong enough that I never had a fuel problem. Only reason I found the in-tank pump at all was because the new 350 I had wasn't drilled for a mechanical pump so I needed to install an in-tank pump.

3tas4me

I am going to try something relatively simple first. I purchased a needle and seat from Cliff Ruggles. His seat has two windows in it that allow much more fuel flow into the float bowl of the carburetor. He tells me that the needle/seat assembly supplied with the carb rebuild kits do not flow enough fuel. Since I did overhaul this carb back in the late 80's, changing this out will be simple and may cure the problem once and for all. Only problem.....I won't know until spring if it is really fixed as car is stored away for winter.

87 player bird

If that doesn't work, a bigger problem(ie:more work) exisists. There is a port under the carb in the intake that can get plugged. Remove intake and run pipecleaners thru every port. Should fix froblem. We call it carburator icing where I'm from. Works every time but a pain in the &@!

ViperineWaters

iTrader: (1)

figured i'd chime in. my car is an '85 L69 trans am (originally) and I encountered a similar problem when I swapped to a 400 sbc, so I dived into research on the issue. it seems the L69 did suffer from vapor lock back in the day, as evidenced by a dealer TSB. I mean no disrespect to sofaking, as he is suggesting you start with the simplest concerns first, which many of us overlook. but the rest of the posts gave me the urge to offer what I found. the dealer fix was the electric pump install, which will add wiring, a relay, and the aforementioned oil pressure switch. this pump was installed as an assist pump, so your mechanical pump is still used. it seems this fix was enacted mainly to cure the excess heat vaporizing fuel in the lines right next to the exhaust. not all of the l69's got this fix, as my '85 did not. when I encountered this problem with my new engine, the first thing I did, to verify the fuel delivery problem, was install a clear plastic inline fuel filter into the rubber line feeding the fuel pump. I forget the part number off hand, but it's a common filter and I think it was referred to me as a VW bug part back when I looked. this will easily show you what your gas is doing when you start stalling. this is NOT the cheap chrome filter you see with a glass tube at part shops, but I suppose that may work as well. once I saw how my fuel delivery was failing, I took a few steps to stop it. first was to wrap the line in a heat shield near the exhaust. second was to add a carburetor spacer (phenolic/plastic was my choice at 1" thick), and continue using the inline clear filter before the pump in addition to the aftermarket edelbrock filter after the pump. while keeping the carburetor cooler is a second fix, I thought i'd mention it since it made such a difference when I starting checking temps with a temp gun and at least adds peace of mind to know i'm not risking boiling fuel right out of the carb. your choice if you're still running a stock setup (especially stock hoods as clearance is minimal). sorry to drag this out, I just figured i'd explain my eventual fix. all starts with a cheap, easy filter.
John

sonjaab

Late 84s and up had both.... a in-tank and block mounted pumps.
It was GMs cure all for vapor lock/fuel starvation in the L-69s.
My 84 does.

You could possibly have a rotted metal line or a cracked rubber hose
connection hidden in the frame or above the fuel tank.
This happens a lot with vehicles driven in winter in snow and salt land.

3tas4me

My car has never seen a winter since the day it was manufactured. I took the carb off the car tonight and removed the air horn. One thing I found was that the float level was far too low. Spec in the service manual is 11/32 +or- 1/32. Mine was somewhere around 17/32. I installed the new needle and seat from Cliff Ruggles and set the float level at 10/32. Pic is of the new seat and you can see the side windows that will let fuel into the bowl faster than the standard type seat.

3tas4me

After thinking about this issue more, I realize that I have actually been having two separate problems. The dying at the top of third gear is a fuel delivery problem, the carburetor bowl is emptying out after sustaining WOT through three gears so it runs out of fuel. I believe that the new needle/seat assembly along with setting the float height correctly will solve this problem. If not, then the mechanical fuel pump is the next cause.

The second problem, which is running poorly for a short time after hot soak is likely the vapor lock problem. It remains to be seen if solving the first problem (which I believe I now have) will also help the second.

3tas4me

Since the carb is off, I thought I would snap a picture of the heat shield that sits between the carb and intake. This shield was installed at the dealer as per one of the campaigns for the vapor lock. This was installed sometime in 1987 shortly after I bought the car. The shield is made out of aluminum. I don't know how much it helps, but I have never removed it. It does require a second gasket that sits underneath the shield on the intake.

RWB____s

I purchased my L69 Z28 new in 1984. It had the very same issue. it would run out of gas going up hill with a full gas tank, unable to pass anybody at freeway speeds with out the risk of stalling out of gas.
I took it to the dealer for warranty repair and they could not find the problem because they were restrained from driving it aggressively enough to recreate the problem because of a 55 mph speed limit they refused to go beyond and the tune up tech could not find the problem buy revving the engine setting in his tune up bay. The engine had to be under load.
As it turned out it took several months and going to a factory rep to find the problem and for GM to come up with a fix. The problem was not a lack of fuel pressure , but fuel volume. I was notified when the dealer had a kit to fix my car. I took it back to the dealer and they had to replace the entire fuel system gas tank forward to the carb. Tank, lines, fuel pumps and lines to the carb. everything except the carb. All under warranty. never had a problem after that. In fact after the car was off warranty I installed a 1st gen HO 350 engine with a 750 Holley carb and had no problems with supplying that engine with the new fuel system fix.

gt4373

Wow,That Had To Be An Extremely Rare Occurrence That The Whole Fuel System Had to Be Replaced.

3tas4me

UPDATE:

Finished reassembling the carburetor on Saturday. Reinstalled it on the car and took it for a ride. Did two WOT runs starting in first gear through 3rd into 4th. First time there was one little hiccup at the top of 3rd, car may have not been completely warmed up by then. Second time, didn't miss a beat, wound it out through 5500 RPM before shifting into 4th. At this time I will say that the running out of fuel problem seems to be cured. I won't know for sure until next spring when we get warmer weather and I can start driving it again. Unsure about vapor lock issue after hot soak as temp was only 10 degrees here on Saturday, not much chance for hot soak in that temp. We got 5 inches of snow yesterday and now the roads are salt covered. Done driving it now until next spring.