Surging during Acceleration

Surging during Acceleration

My new 86.5 is way down on power. This is what I know. The wires, cap and rotor were checked on a scope, all ok. A used LH brain from 928 Int. was installed, fuel pressure checked ok, injector voltage ok, The car starts fine and will pull to red line but, it is way down on power and surges as it tries to accelerate. Could it be plugged converters or as Dave Roberts suggested, dirty fuel injectors? I now can appreciate the simplicity of my 79 Euro.

Tom Knebes
79 Euro 5 spd (old faithful)
86.5 5 spd

Surging usually indicates a lean mixture (not enough fuel for the amount of air). Your mixture should be controlled very precisely by the computer, based upon input from the oxygen sensor.

Some causes for a lean mixture:

Bad oxygen sensor. You can test it pretty easily, if you have a high impedance voltmeter (such as a decent digital multimeter). Find the fairly heavy green and white wires that come thru the floor to the computer, and hook the voltmeter to them. With the engine hot, you should see somewhere between 0.2 volts and 1.0 volts, depending upon the fuel/air mixture. The voltage should change with different throttle openings and loads. If there is no voltage, the oxygen sensor is bad.

Bad fuel pressure. A little tougher to test. There is a test port on the front of the right fuel rail, but it takes a special connector to hook up. You can cut a spare rubber fuel line between the rail and the regulator, and install a Tee fitting for a gauge. Fuel pressure with the pump relay bridged and the engine stopped should be 2,5 bar +/- 0.2 bar, or 36 +/- 3 psig. At idle, you should have about 2,0 bar or 30 psig. If the pressure is low, you can slowly and carefully squeeze the regulator return line closed. Don't let the pressure go higher than 45 psig or so with the engine running - the pump should put out about 60 psig max. If the pressure increases, you probably have a bad regulator. Check the regulator and damper vacuum lines for fuel - there shouldn't be any. If the pressure is good, and you can't find anything else wrong, try a flow test. The pump should put out at least 1350 cubic centimeters in 30 seconds. If not, try a new fuel filter. If the flow is still low, check for a pinched line, bad tank vent system (test flow with the cap off), or other restriction. If you can't find any other reason, replace the fuel pump.

Dirty injectors. Sometimes, Techron will clean them. There are two types of Techron additive - you want the expensive one (so, what else is new?) Additives work best on short trip driving, not long road trips. Bad computer or wiring. Last resort. There are some tests for the computer if you get that far.

A partially plugged cat or exhaust can cause some interesting problems. The easiest test is to put a vacuum gauge on a manifold port. Check the vacuum on a warm engine at idle (probably 15" to 18" mercury). Pop the throttle, revving the engine to about 3000 rpm, and hold the revs steady there. The vacuum should drop momentarily, then climb to a higher number (probably 20" to 25"). If the vacuum stays steady, the exhaust probably isn't plugged. If the vacuum starts dropping after a few seconds, the exhaust is probably partially plugged.

Wally Plumley
928 Specialists

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