High Idle on GM 5.7L TBI Solved
David Allen
Falls Church, VA
all...@mnsinc.com
Subject: High Idle on GM 5.7L - Problem Solved
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Da Problem
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Many thanks for helpful suggestions and insightful comments about high
idle problem on my 88 Chev conversion van with the 5.7L TBI engine.
I wanted to tell you what I did about it, and I am passing tid-bits
from others.
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Cause
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Don Wendel and John DeBelder fingered the cause -- a deteriorated
gasket between TBI base flange and induction manifold.
Don wrote, "When you sprayed carb cleaner around the TBI base you
found the vacuum leak. ALL GM TBI engines are very prone to sucking in
a piece of this gasket and thus causing a hard to spot air leak.
Solution is to replace the gasket."
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Easy Fix
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I had been reluctant to dive in, but after reading the replies and
reveiwing the Service Manual I disconnected the fuel feed/return
pipes, the 4 vacuum lines, accelerator, cruise & trans kick-down
cables, ran out the 3 hold-down bolts and the TBI lifted right off.
More than one third of the gasket was completely gone (mainly along
the narrow faying surface beneath the throttle shaft bushings on each
side of the throttle body. This was the area under the Throttle
Position Sensor where I had sprayed the degummer & noted an idle
reduction.
The rest of the (factory original) gasket was extremely brittle and
badly deformed -- it looked like it was melted at one time (possible
engine heat and/or fuel injector cleaner additives?).
In addition to sealing the periphery of the TBI/manifold interface,
the gasket lies directly beneath the open channels milled into the
bottom surface of the TBI (in front of the two circular induction
ports)which direct intake vacuum to the external accessory ports.
Here, the gasket was sucked up into (and partially obstructing) the
vacuum channels.
The missing gasket sections had been drawn down into the intake
manifold and apparently passed through the engine without
consequence(I hope).
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Results
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A new gasket, a couple of fuel line o-rings, and a new TBI/air cleaner
adhesive foam gasket (total parts cost $8.83) brought the P/N idle to
599 RPM (spec is 600).
Interestingly, the brake pedal came up over an inch, since its booster
vacuum source is the manifold. Vacuum-driven heater/ac controls and
cruise control solenoid are also more responsive.
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TBI Injector Chatter (audible & electrical)
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The engine is so quiet that I now hear other sounds. One, the
injectors are really noisy -- just clattering away. I wonder if that
is normal
The fuel spray pattern is uniform and conical, but instead of a fine
mist there are irregular large droplets and streams sprayed onto the
throttle plates.
Since the van passed emissions test without major change in
hydrocarbon outputs, etc., I guess this is okay; but with 7 years and
83K miles I am pondering replacing the injectors or waiting for
something to break. I would be interested in any pros or cons on
this.
I had noticed that when I tried to monitor the ECM injector signals
with a simple Radio Shack hand-held pulse detector there was a bunch
of electrical noise on the lines. Even at low rates I could not get a
clean pulse.
I assume this is an overly-sensitive pulse detector in a noisy
electrical environment; but I read a separate post by George Bonser
about a sulfur odor in the exhaust, possibly caused by
chatter-inducing back-emf from the inductors in the injector
solenoids.
George wrote --
"There is a problem with "ringing" in the injector solenoid driver
circuit. It does not seem to be "spike" related but a suppression
diode would not be a bad idea. This ringing causes the injector to
"chatter" and makes the fuel delivery difficult to control....
Find a parts jobber that sells Standard(R) inanition components. If
they keep the tech bulletins issued by Standard, there will be one
describing this odor under load condition. (It was issued over a year
ago...."
If anyone can describe these parts and their installation, it might be
nice to know.
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Availability of TBI Rebuild Gasket Kit from GM
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When picking up the parts at a chevy dealer I learned GM has a TBI
rebuild gasket service kit for $42. There were quite a few parts, but
only skimpy documentation.
This appears to be a kit used when replacing the injectors, but there
are other gaskets and washers for the fuel pressure regulator, etc.;
however, apparently none for the throttle shaft where it passes
through the throttle body.
On the 5.7L Rochester TBI 220 the shaft/body seems to be a clearance
fit. I didn't see any bushings or bearings. Mine had no apparent
slop, but I looked closely at this after reading this reply from Mike
Morris --
"My old Volvo wagon with twin SU carburetors had worn butterfly shaft
bushings - I ended up stripping the carb bodies, and using a milling
machine to bore the bodies out so I could press in real bearings,
complete with air seals. inserted sealed bearings." [Whoa!]
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How to Find a Vacuum Leak
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In addition to the obvious Look, Listen and Feel, a couple of posts
mentioned using WD-40 spray instead of degummer around suspected
leaks, (one mentioned lighter fluid to SPEED UP the idle -- Careful!)
and Mike Morris also passed this along:
"My mechanic once found a vacuum leak on my 350 engine around the
intake manifold to cylinder head gasket by using a small pump type oil
can - he just laid a bead of oil along the suspect area, and if the
idle RPMs changed, he knew he had found it."
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All right -- I Will SHUT UP (shortly)
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Thanks again to the folks who were kind enough to respond. This is a
lot of wind (and bandwidth) for a seemingly mundane problem -- mundane
when you know its cause.
The best replies were from rec.tech.autos (expected), and rec.amatuer
radio (also expected -- as a ham I understand the mentality of people
who love to tinker.) I got a few nasties from folks apparently
exercised by posts to "inappropriate" news groups, including a few
lids who were calling for the Network Nazis to descend upon me.
Didn't phase me. Hope they get over it. On second thought, maybe I
hope they don't.
George B
a frequent problem. Fords have a simillar problem with the gasket where the
upper and lower intake halves meet on the 5.0 PFI motors.
The "ringing" is corrected (if you have the problem) by removing the pins from
the injector conectors and soldering a disk ceramic capacitor of a value that
escapes me at the moment but .01uF 50v seems to stick in my mind. You need to
form the leads on the cap in such a manner that the pins can be reinserted
into the connector shell.
This is to compensate for a failing injector driver circuit. The reason that
this is done rather than replace the ECM is because the replacement ECM often
has the same problem!
You are a ham...do you have an O'scope handy? A dual channel unit makes this
easier. Put the probes across one of the injectors, invert one of the channels
and display A+B. You are now seeing what the injector sees...the DIFFERENCE
between the two wires... (-A) + B is the same as B - A .
You will HOPEFULLY see a nice clean injector pulse if you grounded BOTH probe
grounds to the motor (NOTE....THE VEHICLE WILL NOW BE AT EARTH POTENTIAL
IF YOUR SCOPE HAS A THREE_PRONG CONNECTOR...BE MINDFUL OF THIS WHEN IT COMES
TO DROPLIGHTS, CORDS, ETC) if you see ringing after the end of the pulse, get
into your junkbox and pull out a few different values of caps and try
them...NOTE: Do not get too big a value...you do not want to start to filter
out the pulse itself! Try some in the range .001uF to 0.1uF
Good luck!
George Bonser
gr...@cris.com
GBlessing
and take a look. An optical illusion can happen if you don't view them
this way and it can be very convincing. You will swear one of the
injectors is dribbling and not spraying well, put the light behind it and
you will see a real nice cone shaped spray pattern. I have seen this fool
many a mechanic, hell I worked with a guy at GM that replaced 3 injectors
and was cursing when I came over to see what was going on. I put the light
behind it and the perfect spray pattern was obvious, so was his dumb look.
J Blessing ASE Master Tech