A layer of rust in the bottom of the tank is bad juju. When you start throwing the car around the track sloshing fuel will stir up the rust. It will then collect on the inlet screen of the pump and starve the engine for fuel. Suction will keep the rust on the screen until you shut off the engine.
More than half of the work involved in cleaning and sealing a tank is removing and re-installing the tank. Which you have to do whether you are cleaning the tank or replacing it. Cleaning and sealing the tank can be done over a weekend pretty easily. Especially if you drop the tank Friday evening. If your tank is otherwise in good condition cleaning and sealing the tank is a lot cheaper than a new tank. My experience with a rusty tank are:
A twenty year old car with a steel gas tank can have a surprising amount of
rust in the tank. That rust can and will clog the pump inlet screens and
filter and cause the high pressure pump to have a short life. While a
new tank will certainly fix that, you can clean and seal the tank for a lot
less money.
My 87 325is suffered from this. The car ran fine on the street but the first
time I took it to the track for a test & tune it was as if the car had a soft
rev limiter that decreased by 500-700rpm per lap. Some particulate matter did
come out of the tank when I drained the gas, but with the in-tank pump out I
could see a layer of crud in the bottom of the tank that was about an 1/8"
deep. On the track that stuff got stirred up, clogged the inlet screens &
filter, and starved the engine. It also ate up the internals of the high
pressure pump. Rust is a fairly abrasive material.
The first attempt at a fix was to have the tank acid dipped and install new
pumps & filter. Either they didn’t leave the tank in long enough or the acid
was a bit depleted, but the result was less than satifactory as the next time
I had the car on the track the high pressure pump started buzzing and I ran
into the same starvation issue.
After taking the tank out again and sloshing about a gallon of fuel around I
got loads of rust out of the tank. The new filter was heavily loaded and the
inlet screen of the high pressure pump was also loaded up. And the cross-over
pipe was completely plugged. This time I was determined to get all of the rust
out and follow that up with sealant to lock down anything that was left and
prevent further rusting. I used a gas tank sealer kit from Eastwood, but went
a bit further than they suggest.
The tank, having been recently acid dipped had no varnish in it. So a simple
wash with a strong TSP solution removed the remaining organics. I made up a
simple plug for the hole where the in-tank pump mounts that could easily be
removed. Think of a round disk of plywood with a bar on the bottom and a couple
of screws to snug the bar up. Drop that into the opening, rotate slighty to
engage the lock ears on the tank and tighten the screws. The remaining
openings were closed up with pieces of a freezer bag held in place with rubber
bands.
About a gallon of diluted acid in the tank was about right. That was enough to
slosh around well but not so much as too make the tank to heavy. I used an acid
mix sold for cleaning masonry that’s composed of Hydrochloric and Phosphoric
acids at about double the normal dilution. Over the course of a couple of
hours I’d slosh and flip tank every fifteen minutes to wet all parts of the
tank with the solution. At the end of a couple of hours I drained the tank and
disposed of the pretty well spent acid by reacting it with limestone gravel.
To flush the tank I made up a right-angle spray nozzle with plumbing fittings
and a 1/4" hose barb. That allowed the nozzle to be inserted into the tank and
a high velocity stream sprayed into all corners. This was done with the tank
at about a 30deg angle (drain plug hole down) and the fluid collected into a
bucket. When the water ran clean and no more loose junk came out I repeated
the acid treatment and flush. It took all of one Saturday and part of Sunday
to get the tank to the point that no more rust could be seen and nothing was
being flushed out of the tank.
The last steps were to use the acid etch from the Eastwood kit, flush with
acetone, and apply the sealer. Before putting the sealer in the tank I removed
the cross-over pipe and sealed those openings. And after the sealer had been
applied and before it had a chance to harden I blew out those openings and the
tubing that runs across the top of the tank with compressed air. After drying
for a couple of days the tank was reassembled and reinstalled in the car.
Once sealed the inside of the tank had a fairly uniform white coating. I did
see a few specks of matter that I wasn’t able to flush from the tank, but
those were well locked in place by the sealer.
Cleaning and sealing the tank is pretty labor intensive. But at a total cost
of less than $80 it is a lot cheaper than a new tank (~$400 for this car).
