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Intermittent stumble, intake backfire on '94 Suburban
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CarGuru > Technology > Intermittent stumble, intake backfire on '94 Suburban 3 March 2005 17:13:04
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Intermittent stumble, intake backfire on '94 Suburban

Guest 28 February 2005 02:38:00
 Symptoms: When warmed up, random intermittent stumble, most noticeable
on acceleration, sometimes with backfire through intake manifold.
Feels like the whole engine suddenly shuts down for 1/4 - 1/2 second,
repeated a few times over the course of ~5 seconds, then no problem for
another few minutes. Never happens at idle. Tachometer "twitches"
+/-100 rpm or so irregularly, continually. Slightly rough idle; has
never stalled. No diagnostic trouble codes (except DTC 12, "system
OK") are in evidence when I get the engine to blink them out on the
"Check Engine"-type light.

Just replaced the O2 sensor (first replacement in 203,000 miles),
ignition module and ignition coil. New plugs, wires, cap, rotor
installed 23,000 miles ago. Spark plugs have always been clean when
inspected and continue to look excellent. Bending and jiggling the
coil wire manually at idle doesn't cause anything. Pickup coil
inside distributor had been replaced along with ignition module 30,000
miles ago to cure the only no-start in 203K miles. The pickup coil has
a metal shield around it that can be rotated back and forth about a
half inch or so by hand. Pickup coil resistance is in-spec. Generous
dose of "dry gas' fuel additive dumped into a full tank several
weeks ago to remove water if there were any present.

Using a relatively slow laptop oscilloscope (and sampling even more
slowly at about 20 samples/second so I can stop a trace and save it
just after I notice a stumble), I find the Throttle Position Sensor
seems to be behaving the same during episodes as when normal. The MAP
output during normal operation seems to behave like I'd expect. Fuel
pressure downstream of new filter is OK and constant at 12 psig through
episodes. EGR can be worked open and shut using a vacuum source with
engine off.

I haven't even narrowed it down to fuel or ignition yet. Is there
any way to know if the 2 throttle body injectors are actually spraying
fuel at all times, for instance by measuring their current flow?
(I'm not buying a borescope, lights, and video camera to film the
spray pattern as I drive down the highway at 55. : )) Some way to
monitor the injector pressure itself?

How does an engine backfire through the intake, anyway? There's
always a flammable mixture in there so the ignition source must be
provided somehow during an episode. The service manual, when referring
to stumble on acceleration, points one toward fuel system first, then
ignition. When addressing backfires (exhaust or intake), fuel
disappears off the list of things to check. All I can imagine is both
valves open at some point with unburned fuel in-between and therefore a
combustible path between glowing carbon cinders in the exhaust system
and the intake manifold.

I don't think my laptop scope is fast enough to get at the details of
either the primary or secondary ignition waveforms. I've heard that
one can tell if a given cylinder is running rich or lean from the
secondary voltage waveform. ! I could buy a supposedly suitable scope
for about $600 from Picotech, with a little extra for current clamps
and secondary voltage pickups. I'm almost thinking that this would
be preferable to replacing parts one by one, if I could convince myself
I'd find the culprit and replace only it.

I imagine the Powertrain Control Module would be the most expensive
thing that would cause this problem; I don't know how much they sell
for. Can I set up the ignition system to run self-contained within the
distributor for a test drive to see if the problem goes away? Other
suggestions to troubleshoot the problem?

Add comment
Guest 28 February 2005 03:28:25 [ permanent link ]
 I would recommend you perform a "leak-down" test - don't bother with a
compression test - because it sounds as if you could have an intake
valve that is letting compression get by.

Add comment
Guest 3 March 2005 17:13:04 [ permanent link ]
 The problem is intermittent. Wouldn't a valve problem (except if it
were sticking open) be evident all the time? How would one diagnose an
intermittently sticking valve anyway?





sallycode@sbcglobal­.net wrote:> I would recommend you perform a "leak-down" test - don't bother with
compression test - because it sounds as if you could have an intake> valve that is letting compression get by.

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CarGuru > Technology > Intermittent stumble, intake backfire on '94 Suburban 3 March 2005 17:13:04
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