Intermittent stumble, intake backfire on '94 Suburban

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?

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.

> wrote in message
oups.com...
> 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

Does the check light come on when you turn the key to the on position? if NO
light -- R/R computer!

> OK") are in evidence when I get the engine to blink them out on the
> "Check Engine"-type light.

I'd take a look at the ignition system >Ignition Module or perhaps a cracked
pickup<



> dose of "dry gas' fuel additive dumped into a full tank several
> weeks ago to remove water if there were any present.



That stuff will ruin your engine in no time -- I recommend against that crap
unless absolutely necessary or you don't mind an early engine retirement!


> 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.

Do this >> Use a timing light -- Engine: ON - point light at timing marks --
then with other hand apply throttle slowly -- timing marks should slowly
advance -- if erratic -- ignition system or perhaps a timing belt issue!


> How does an engine backfire through the intake, anyway? There's

How does one burp -- is similar! <grin>

Misfire, worn timing belt, ignition issue or a fuel issue as well will cause a
backfire!

BBA

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?





wrote:
> 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.