928autobahndreamer

Not sure what is going on, but I wanted to check in with the collective wisdom of Rennlist.

For the last few years I have noticed after a hard drive the fuel pump will start buzzing pretty loudly. Initially I thought this was the fuel pump starting a slow death. So, I changed out the fuel pump and fuel filter. (note that I do not have an intank pump only the screen, which was also changed.)

The visual inspection inside the tank with a flashlight looked clean.

Anyway, this made ablsolutely no difference as the noise continued unchanged. This only happens after the car gets "heat soaked" when the temp gauge gets to just below the second line.

The other day I was out running errands around town in 95 degree heat and had a lot of stop and go driving. When I got home the fuel pump was doing its usual buzzing. However, when I turned off the car, I heard a gurgling sound coming from the back of the car. The sound you get when you are pouring a large amount of liquid out of a narrow necked container, (you know glug, glug)This lasted about 20-30 seconds.

I am wondering if anyone has experienced anything like this? Could this be something keeping the fuel from circulating properly? (ie pinched line or some other obstruction). Is it possible the tank is not venting properly? Or is the fuel just heating up to the point that it is boiling? Unsure what to think about it.

Thanks

Leon Speed

If the tank is not venting properly, you should be able to check this by loosening the gas cap - you'll hear the air being sucked in.

davek9

Were you running the AC? There is a Fuel Cooler on the Firewall that cools the fuel b4 it returns to the tank, but the AC need s to be on and working
I have never seen or heard what you have described.

928autobahndreamer

I did not try this last time, but have refilled when hot and have never heard any hissing on removing the gas cap.

928autobahndreamer

My AC unfortunately does not blow cold. I therefore have not been using it. Windows down and full speed ahead has been my AC.

However, now that I am moving to Texas, my priority for the AC has moved up dramatically.

dr bob

Prior to the addition of the fuel coolers on injection cars, heat-soaking and boiling fuel was a common and serious problem. Early CIS cars of various pedigree (Saab, BMW, MB, etc) had this problem a lot when ambients temps were high. Bosch initially prescribed higher primary fuel pressures and bigger pumps to mask the vapor-locking problem that would cause the car to stall or at least run poorly. But they were chasing a symptom rather than the core problem. Fuel was circulating through the hot engine bay, often through metal fuel rails or plumbing sitting right on top of the hot engine. CIS routed fuel through the warm-up regulator that was heated by engine coolant as part of normal operation. The HDPE tanks do not have much heat-shedding available. So extended hot-day driving in CIS cars often included boiling the fuel. Not good!

Solution is to get the AC functioning correctly, so the returning refrigerant can carry some of the heat from the returning fuel flow on its way back to the fuel tank.

I still have a couple of those hi-perf CIS pumps in my spares collection. Got a compressor CIS car? I have the right pump for you!

namasgt

I have the same problem with my 88, AC not working. Someone here was saying that I could put some insulation on the fuel rails and lower the heat transfer amount from the engine bay. I have not installed the fuel rail covers ever since the engine rebuilt.

17prospective buyer

Also would help to take your intake assembly off and the MAF to inspect the insulation around the fuel cooler. At this age, most of the original insulation has probably been transformed by heat and aging into a solid foam like material. Mine crumbled right off. I pulled it off, cleaned it thoroughly (capping all fittings), then reinsulated it with the foam insulation used for home plumbing, the inside diameter of the wrap wrapped right around perfectly. I then used reflective duct tape to wrap tightly around the fuel cooler.

dr bob

I added insulation back on the rails, more for the sound than for thermal issues. There's no good way to completely insulate the bottom of the rails where radiant heat from the head would propagate directly to the rail. I added a few layers of 1/8" thick foam tape under there, but I'm not so sure it does a lot for the total heat transfer. Bad is that there's lots of fuel piping exposed in the engine bay besides the rails. The regulator and dampers bolt to hot metal parts, for instance, and have fuel flowing through them continuously. The comment about the insulating the cooler itself is good. I added a couple wraps of thermal insulating tape around that canister, followed by closed-cell foam that matches the original in both color and texture. The tape is doing all the real work there, foam on mine isn't much more than decorative. Note that this only helps slightly with heat absorbed into the A/C side (shell) of the little cooler, and won't help much at all unless the AC is in service.

928autobahndreamer

Hmmmmm...........I have done a top end refresh on this vehicle already. The fuel rails are covered with an almost tar like material. I don't think it is the insulation that was originally on the rails. It is black and I can't help but think that this is holding heat in the rails instead of letting it disperse. So, I think I will scrape all of it off and try the foam and reflective tape. I may also try wrapping some of the expose metal fuel lines and cooler with said tape.

I also think that I will trace the fuel lines to make sure that there is not a kink or section limiting flow. I am remembering that when I changed out the fuel lines after I first got the car that there was one particular area that made me a little nervous. The fuel line that loops back near the throttle cable pulley. The line that I put in that area looked like it would tend to flatten at the area of the tightest bend. I may go ahead and look to replace this with the factory shaped piece just to make sure that I am not limiting flow and causing the fuel pump to have to work extra hard or cause it to cavitate.

Finally, I will get going on getting that AC fixed. This will also have to include fixing that diaphragm under the dash that is keeping my heater valve from closing properly. Looking at receipts from the PO, thousands of dollars have been spent on the AC before my purchace. Converted to R134 and at least 2 previous compressor replacements. So, hopefully I will be able to track down what is wrong and get things working like they should.

I'll keep you posted on what I find out.

928 at last

If you have an IR heat gun, shoot the tank and filler (just inside the door on the fender). Last summer mine was in the 100 degree range after a commute, and the fuel pump was not happy. I've found that the fuel cooler is critical in anything over 80 degrees. Now, like you, the AC has become a priority item.

Bill Ball

I think you have correctly ID'd what is happening. Your car might be one that would benefit from an in-tank pump. It was part of the solution, along with the fuel cooler, for just this problem.

I'll just relate that we had dramatic fuel "boiling" issues in George Suennen's car with multiple reasons why. One of the causes of this phenomemnon is that a high proportion of the fuel that is pumped up to the engine compartment recirculates back to the tank, diverted by the fuel pressure regulator. This transfers a good amount of heat from the engine compartment back to the tank. George absolutely magnified this problem by installing a VERY high flow fuel pump, designed to supply enough gas for 1000 HP motor. George's supercharged motor was producing over 600 crank HP, so he thought the pump was a good idea. However, under almost all driving conditions other than WOT his motor was producing no more HP than yours or mine, so a HUGE majority of the gas was being returned to the tank only to be pumped out again and recirculated over and over. The other factor this high output pump created was PRESSURE in the return lines from the excessive recirculation and a large pressure drop in the fuel delivery lines. Under normal conditions there is NO pressure in the return lines and minimal pressure drop in the delivery lines. This heated the fuel even more with the pump working against resistance.

We discovered this after experiencing a progressive drop in fuel pressure and leaning out of the fuel mixture, accompanied by complaining noises from the fuel pump. We installed a temperature sensor in George's fuel cell and could see that as the fuel temperature rose above 125F, the pump would start complaining, pressure would drop, and when the fuel pressure dropped below 30 PSI the injectors would stop firing and the car would stall. These temperatures were reached on 70-80F degree days but took several hours to develop and the final symptoms would not appear unless we were at high desert altitude. Altitude contributes to lower the temperature at which liquids vaporize. We installed an electronic fuel pump speed regulator that lowered the output of the pump when just cruising by some 50% but that proved ineffective.

Another contributing factor was the fuel cell George installed in place of the stock fuel tank. It was closer to the exhaust pipe and did not have heatshielding.

And the pump itself, a nice looking Aeromotive unit, was installed externally with NO in-tank priming pump. And it SUCKED the gas out of the TOP of the fuel cell, another design feature that enhanced the development of vapor lock. The supercharger itself was not a factor as the problem occurred when cruising off-boost at lazy highway speeds.

It took us a while to piece all this together from the initial observation of fuel mixture leaning out. And the solution required addressing all the contributing factors. Initially we got rid of the 1000 HP Aeromotive fuel pump and installed a Bosche 044 in its place. Great pump! It would continue working as the gas heated up and would not fail until it reached almost 135F. Can you imagine that - the gas was 135F. That's HOT! We had to get the fuel temp down. We doubled the size of the fuel delivery and return lines by running a new large delivery line and using the stock delivery line as a second return line. We insulated all the lines in the engine compartment. We installed heatshields on the fuel cell and wrapped the exhaust pipes in that area. Then we installed an 044 in-tank in parallel. That became the main pump, and only during WOT runs George would switch on the external pump.

All of that solved the problem. Gas temps would still rise, but not exceeding 120F say when climbing in the mountains. The 044s tolerated that without a problem and with this setup George was finally successful in completing the Silver State Challenge at an average speed of 160MPH, topping out at over 200.

928 at last

Has anyone ever considered doubling up on the fuel coolers? or perhaps increasing the capacity?

dr bob

The fuel cooler is only effective with the AC system operating correctly. "Doubling Up" the coolers isn't necessary with proper AC function, and without that AC function any increase in cooler capacity would be fruitless.

I'm not quite ready to risk having a direct fuel-to-air in front of the AC condenser, and that's what might be the "solution" with no separate heat-removal capability. In any accident, that exchanger would be way too exposed.

Bill Ball

The stock fuel cooler (a section of the return fuel line welded into the low pressure AC line) would be rather hard to double and is probably adequate as is...as long as your AC is working and turned on.