From: John De Armond
Subject: Re: 454 backfiring problem
Date: Tue, 07 Mar 2000 02:26:52 EST
> Have a 1990 Pace Arrow with the 454 throttle body fuel injection.
> When accelerating under moderate throttle from 30-35mph it will
> backfire (it sounds like an intake, rather than an exhaust backfire)
> at 44-46mph. This will happen almost every time. If using a more
> aggresive throttle, it does not happen. It never happens any other
> time or other speeds. Have the latest EPROM, there are no computer
> error codes. In shop testing shows no electrical problems.
> One mechanic suggested a possible 'tuliped' valve, probably caused
> by overheating at some point. I have NEVER run with a overheat situation.
> His suggested fix was a $6000 replacement engine ! He said if one valve
> has failed, others are probably close behind, along with damage to the
"tuliped" isn't a very precise term but if he means that the valve
stem has stretched or the valve has sunk into the head because the
rim got too hot and bent, this is easy to detect by measuring the
valve stem heights by simply taking the valve cover off, removing
the rockers and checking the stem heights. A stretched stem will
protrude farther than the rest.
This might be the problem but I'd look elsewhere. You didn't say
how many miles you have on the rig. If it's moderate to high
mileage, it might have burned a valve. This is easy to detect.
Disconnect the BATT lead from the HEI distributor and crank the
engine. The compression strokes should sound uniform, that is, each
"chug" should be the same. If the starter sounds like it skips over
a cylinder (speeds up, higher pitch), then a cylinder has low
compression for some reason. It could be a bent valve, a burned
valve or ring problems. A compression check should show the culprit
cylinder. If you find a cylinder with low compression, squirt a
couple of ounces of oil into the cylinder. Rerun the test. If the
compression comes up, the culprit is rings. The oil will
temporarily seal worn or broken rings. Everything mentioned so far
can be done with the engine in the rig and without having to take
anything apart other than a valve cover and rocker shaft.
Check the timing, paying attention not only to the static advance
but also how much total advance you get with the vacuum advance
activated and with the centrifugal advance fully in. I'm assuming
the engine has HEI ignition and a distributor and not DIS.
The first thing I'd go looking for is something to cause the engine
to be lean in the midrange. The ECU won't necessarily throw a code
if the leanness is minor. a vacuum hose that has blown off from the
backfires is a likely culprit. Also take a look at your vacuum
brake booster if your rig has one. Multiple backfires can damage
the booster and let air flow through it continuously. I really
suspect an air leak because you say it happens only when you slowly
roll on the throttle. Rapidly opening the throttle will give enough
acceleration enrichment to cover a small leak. Other causes of
leanness include clogged injectors, low fuel pressure and
mis-adjusted throttle position sensor. The ECU can't detect any of
these problems unless it is so lean that it can't go closed loop.
Last but not least, since you mentioned that you have the latest
EPROM, this indicates that you've changed out a CALPAK EPROM. Make
darn sure you have the right EPROM for your particular engine. Your
GM dealer will be able to tell you what CALPAK EPROM number is
correct for your VIN.
I'd certainly not give even a thought to a replacement engine or
heads without a LOT more troubleshooting.