Engine

Basic Diagnostic Steps

The Problem:
Been living with the following symptoms for quite a while now-
professional attention hasn't led to a fix:
Performance is extremely erratic- sometimes soft enought to get blown
away by an accord, then again the other night I floored it (in second)
at sixty and did some serious tail wagging as the tires lost it.
After its warmed up, the idle hunts a bit and the idle itself is a
little rough- not a genuine "skip", but not right either.
The problem seems progressive- ran O.K. (but power waslow) after my
mechanic diagnosed problem as & replaced fuel pressure regulator,
vaccum leak(s), and throttel positon sensor (can you see goose hunt),
all of which were replaced. Now its back to above symptoms, beginning
to pop on deceleration, and has a worsening surge when cruising at or
coasting down from 2500-3000 rpm.
Mechanic also said that voltage to fuel pump was low at 9V and that it
was only putting out 2.2bar. But I'm thinking that is a separate
problem because the performance fluctations occurr while the pump is
in this condition.

The Reply:
First of all, you should find a mechanic that REALLY knows
LH-Jetronic instead of second-guessing at your expense. You didn't mention MY. I would
check the following:

. Do a compression check.
. Verify all vacuum connection. Are your vacuum servos operational and not leaking?
. On the fuel pump getting only 9 volts. This is definitely a problem. It is reasonable
to say that if you have a voltage drop of 30%, fuel delivery drop by the same amount.
Re-verify voltage level along the fuel pump circuit. The fuel pump relay's contact is
probably pitted.
. Fuel delivery system check:
(1) Measure fuel pump head-on pressure. This is the raw pressure generated by the pump
with the engine NOT running. Compare this to spec.
(2) Measure fuel pump running pressure. This is fuel pressure with engine running.
(3) Then finally, measure residual pressure. This is the pressure retained when ignition
key is off; engine and fuel pump NOT running.
. Does your MY have an in-tank fuel pump as well? If so, is it running?
. Check for proper operation of fuel injectors. Use a combination of a stethoscope and a
"noid" light. Check the condition of the injector seals. Any "False" air will throw off
the brain.
. Is your crank position sensor working? Erratic operation of sensor will cause misfiring
and intermitent start and performance problem.
. Per one of my previous post, check alternator for AC ripples. This is definitely a
no-no for modern day electronics.
. When's the last time you've checked the timing belt? A stretched belt will cause
inaccurate valve timing and more seriously, BENT VALVES.
. You might have one or more burned exhaust valve(s); hence detonation. Inaccurate valve
timing will cause this too. A leak-down test will uncover this.
. All electrical connectors corrosion-free? Especially the brain circuit.
. Battery OK? Low voltage will cause erratic idling. Generally, a low voltage condition
will cause the engine to run rich.
. Do you have cracked or leaky spark plug wires? What's the primary and secondary
voltage?
. You can do a resistance check on the MAF, but a direct swap is much easier.
. Compare your Temp sensor resistance with a known good one.
. When's the last time you've changed the O2 sensor? It might be "lazy" and this will
affect your idle rpm.
. What about your idle air control valve? Is it gummed up?
. Is your cold-start injector dripping? This will cause erratic idle.

I can go on and on.. but you gotta make sure that your fuel delivery and electrical system
is up to par first. Good luck!

Vince Yu
'83 928S
'78 928