TransAm Rocker Poker Issue
Robo-Tech was called to a shop with a 19 98 Pontiac Trans Am, and a 5.7 liter engine,
having a misfiring issue.
The shop replaced parts gallore, including the plugs, wires and fuel filter, which was
totally unrelated.
When the tune up didn't work, the shop performed a fuel injection cleaning and de cocked the
throttle body.
Finally, they realized that the missing cylinder was number 5 and took a compression reading,
which proved fine.
It was at this time that the shop decided to call me in for assistance.
As soon as I got to the shop, I questioned the tech for his input on the matter.
He said that other than the missing number 5 cylinder, the engine had power.
He also pointed out that when he removed the number 5 spark plug, it appeared to be cleaned.
He took a compression test, but found compression to be fine, so gave up on the matter.
I started my diagnostic by performing a compression test, but not with a compression gauge.
This engine was totally sealed.
I wanted to use a fast way to determine engine compression, or at least have a ball park
figure.
So, I decided to use my high current clamp on, Amp probe.
The Amp probe is hooked around the battery negative cable, and the engine is cranked.
With each revolution of the crank, the start has to exert more power to move the piston
towards top dead center.
As this happens, the extra current is captured by the Amp probe, and then graphed by the
oscilloscope.
Sure enough, compression was right on target.
So, my next step was to just do a quick scan tool code retrieval to see if cylinder number
5 popped up.
As the tech said, I got a code P0 305, or cylinder number 5 misfire.
I did see that the fuel trims were slightly over, or at about 14%.
This indicated that the nature of my problem was that of excess air.
As tight as the engine was, I didn't want to do major disassembly just to prove the
intake manifold.
I decided to perform an ignition waveform analysis, with my trusty portable engine analyzer.
My unit had a special adapter for coil on plug units or hard to reach ignition, which
consisted of separate green antenna type pick ups for the ignition signal.
This adapter needed no disassembly of the coils.
I did a capture, and immediately saw that the ignition wave for cylinder 5 was faulty.
This was the first indication I had of the issue, and the slightly lean condition.
The waveform also showed that the ignition coil was releasing plenty of kilo volts.
The shorter wave spark duration confirmed what I already knew, that the mixture was
lean.
There were a couple of possibilities, like a clogged injector, defective valve or spark
plug.
The plug and the valve were fine, since I had done the compression test using the Amp
probe.
This test is not exact, but it tells you if you have a significant low cylinder contribution.
The other possibilities were intake gasket, so, I decided to smoke the intake manifold.
I pressurized the intake manifold and could not see a leak anywhere.
A gasket leak that large would definitely show up on the smoke.
So, what could possibly be the issue?
What else was there to check?
I decided to make prove the intake gasket theory, by injecting some propane at the throttle
body.
This was a simple enough test, and would prove the intake gasket.
I used a simple propane torch, at the throttle body.
Then, as I introduced propane, to my amazement, there was no change in the engine miss.
What on earth was going on here?
I had spark, compression, and now fuel with the propane.
All of a sudden I remembered what the tech originally said, that the number 5 plug was
very clean.
I made a decision and told the tech to remove the valve cover to inspect the rockers.
I had seen issues with rockers before, on the exhaust valve that did not show up on
compression.
After removal of the valve cover, I immediately saw the problem.
What a shock?
This was incredible!
The exhaust rocker had come loose and flipped slightly to the side, rendering the cylinder
like an air pump.
hence the lean condition.
This engine had injector kill or shut down feature, so, there was no fuel entering the
cylinder and no converter damage.
Since it only affected the exhaust valve, it still showed compression, but it was a
basic air pump action.
No damage was done and the rocker was replaced.
The vehicle was never seen again.
This case showed the value of following a specific diagnostic path.
We've said it before, diagnostic procedure beats symptomatic diagnostic each and every
time.
Hope you learned from this one.