Accumulators for EFI systems

[sowelled]

Greetings,<br><br>First, the question:<br><br>Do you know of any EFI car that operates at 28-30 psig that uses an accumulator<br>in the high pressure side of the fuel feed system?<br><br>The background:<br><br>I have a 76 Jaguar XJ-S with hot start problems. On a warm day, upon restart after<br>a shutdown of perhaps 30 minutes to an hour there is a bit of rough running<br>and popping (mild backfire). In extreme cases it is really hard to get going, like it's<br>running on 3 (out of 12) cylinders. After a bit it clears up and runs fine.<br><br>The car has a Bosch-Lucas D-jetronic EFI system.<br><br>I have done a lot of research on this, and a lot of work on the car, like rebuilding<br>most of the fuel system (all but the pump, actually). I am pretty sure it's due to vaporization<br>in the fuel rail. I've done lots of tests and am pretty sure that what happens is that<br>during heat soak back the fuel rail gets hot, the fuel expands, and is released through the <br>regulators (see below for the plumbing details) as they maintain the 30 psi regulation pressure.<br>Then, as it begins to cool, the fuel contracts, causing the pressure to drop. During the period<br>when the pressure is low and the engine compartment is still moderately hot, vaporization occurs.<br><br>Now, in a car with a single rail and single regulator, starting the pump would quickly clear the vapor.<br>However, the early XJ-S cars have 2 rails, one for each bank, each of which is shaped like a race track.<br>Fuel feeds in at the middle of the outside straightway, and ingectors tap off along the inner<br>straightway. There is a regulator on each rail, taking return flow from the middle of the inside<br>straightways. Consequently, there are many ways for the fuel to flow, and if one path has vapor<br>in it, it will offer a higher resistance to flow and is therefore in some sense blocked, i.e., a vapor-lock<br>of sorts.<br><br>To try to fix this I first&nbsp;&nbsp;rebuilt the fuel system to be absolutely sure the rail pressure was not decaying<br>down due to check valve or regulator leaks, or external leaks. Both regulators and the check valve were<br>replaced, as was all hoses, and the injectors were refurbished professionally. More recently,<br>I have installed a air temperature thermostat to keep the auxiliary fan on after shutdown. It is currently set<br>at about 135-140 degrees F. After driving on an 85 F day and parking in a closed garage,<br>it will cycle on and off for an hour or more. I believe this might have fixed the problem, but have <br>not driven enough to be sure.<br><br>If the problem persists, the next thing I want to try is to install an accumulator near the fuel pump.<br>This will keep the regulators from releasing fuel during heat soak back, thus preventing rail<br>pressure decay during the cooling period. <br><br>So, the question to you is where can I get such an accumulator? I have learned that Jettas and <br>a lot of other late 80s cars with CSI fuel injection do use accumulators, but these operate<br>at 80 psi. Consequently they MAY be useless on the D-jetronic systems at 30 psi. <br><br>I'd appreciate any and all discussion on any of the points above, or related matters<br>

 

[sowelled]

Since the original post I have in fact installed an accumulator<br>from a 1980 VW Rabbit. It does indeed function as an<br>accumulator in my system, storing about 1/2 cup of fuel.<br>So fan, it seems to have cured the symptoms, but only<br>time and more hot weather driving will tell.<br><br>Ed Sowell<br>

 

[gbh]

Hmm... does the car use an electric or mechanical fuel pump? If it's a pressure problem, I don't quite get how an accumulator (which adds no pressure) will help. A better way to route it would be to route the outlet of one loop to the inlet of the other, and use a single regulator. One inlet, one outlet. That might help ensure that the pump pushes fuel through the whole rail system, instead of only through half of one loop. There'd be some delta P across the plumbing from inlet to outlet, but it may do the trick.

 

I can relate to sowelled problem with his VW. Replacing the fuel pump (check both the lift pump in the tank and the large pump under your car) has always solved the problem. All three of my previous VW Jetta's (Bora) had problems with cutting out in really hot conditions esp when stopping at traffic lights.I would recommend you look at your pump, test outlet pressure.