Posted by Alan Bowes on November 08, 1997 at 13:52:59:
In Reply to: engine problem posted by Jack Cherry on November 08, 1997 at 00:48:07:
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Hi Jack,
Garrison's comment points out an interesting possibility: an overly lean condition due to increasing vacuum in the tank and fuel delivery system.
You didn't make it clear whether the backfiring was through the carburetor or the exhaust. This has implications regarding spark timing. In either case, however, an overly lean mixture or improper spark advance (or some combination of those) could be the cause.
There are several possible causes for an overly lean mixture or incorrect spark advance. If your timing is already incorrect, anything that would cause too much or too little advance could result in a loss of power or backfiring.
As Garrison mentioned, fuel tank vacuum is one possibility leading to an overly lean mixture. You probably have an evaporative control system on the vehicle that is designed to reduce gasoline vapor emission into the atmosphere. Vapors are stored in a charcoal-filled cannister and sucked into the intake manifold to be burned. If this system is malfunctioning, it could conceivably cause a buildup of vacuum in the fuel tank and fuel delivery system, cutting fuel flow to the carburetor. Get a system schematic and check it out.
Other possible causes:
There may be a vacuum spark control (vacuum advance) system that employs a temperature-controlled ported vacuum switch (PVS) that switches the vacuum advance on the distributor to an alternate vacuum source (direct intake manifold vacuum) when the engine temperature is too high. This increases the spark advance and allows the engine to cool down. If the PVS is faulty, it could be applying the extra advance when it's not supposed to, or preventing adequate spark advance under load/acceleration. If your timing is already off, the effects would be exaggerated. This system also may employ a vacuum "amplifier" to increase vacuum signal strength, which may change vacuum routing somewhat, but in this case both the vacuum amplifier and the PVS may be suspect. However, some Dodge trucks use a slightly different system that has a temperature-controlled valve in the upper radiator tank that controls the EGR valve instead of affecting spark advance to effect an engine cooldown.
Some Dodges also use an Orific Spark Advance Control (OSAC) system to control spark advance to reduce nitrous oxide output. This system delays the vacuum signal to the vacuum advance. The delay depends on a small, calibrated orifice in the OSAC valve unit. Some OSAC units have temperature sensors located either inside the air cleaner or near the OSAC unit. Above a certain temperature (depending on the location of the sensor), the valve will move to a position that causes the vacuum signal to pass through the orifice and be delayed. Below that temperature, the valve opens (bypassing the orifice) eliminating any delay. In other words, there is no vacuum delay at lower temperatures. If this sensor or valve is malfunctioning, it could seriously affect spark timing, especially during rapid changes in throttle position and load. If you have the detailed literature (shop manual), this valve can be tested with a vacuum gauge.
The EGR system is probably not the culprit, since if it were sticking open, the engine would be running very rough at idle. However, some EGR systems are tied in with the PVS spark control system, so it could be a factor in faulty spark advance.
A malfunctioning power valve in the carburetor can result in an overly lean mixture under acceleration/load. It is possible, though not too likely, that excess heat might affect the operation of the power valve.
Hmmm, what else? If you have a marginal or clogged fuel delivery system, it may not be able to keep up with the engine's demands. When you let the engine idle for a while, it would give it a chance to get "caught up" and fill the carburetor bowl. When you start driving, the engine might consume fuel at a slightly higher rate than the fuel delivery system can provide it and eventually runs short. Idling for a while refills the carb, etc. In this case, check out fuel pump pressure and pumping volume, lines, filter, pickup, etc.
There are probably some other possibilities as well, but these are fairly common sources. Ported vacuum switches are known to fail or malfunction quite often, especially as they get older. It's not a bad idea to go ahead and replace these just on general principles to avoid problems later on. Check ALL vacuum lines for cracks or loose connections. Check your timing with a timing light according to manufacturer's instructions, and make sure the vacuum advance is working correctly.
Have fun,
Alan
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