A Real Challenge!

I’m getting multiple misfires on a scan tool. Here are the facts to my problem.

1999 Acura CL 3.0L

300,000 miles (yes, that’s 300,000!)

Car’s in beautiful shape.

Rough idle when cold and loss of fuel economy (28 to 25 mpg).

Misfires in 5 of 6 cylinders, and multiple misfire code

Will “pop” code after idling a few minutes. Always #2, then #5 or #6. Eventually all. If I clear codes while driving, they will not come back on. Will come back on the next time I start up.

Previously replaced Cat Converter because popped code, however, prior to the misfire codes.

No other codes ever recorded.

Replaced fuel filter and cleaned injectors. Fuel pressure to spec.

Replaced plugs, wires and coil.

Vacuum at 20-21 Hg.

Cylinder pressure 120-160 psi (depending on how many times engine cranks).

Performed shop manual test on EGR Valve. OK.

O2 Sensors appear to be reading properly.

MAP Sensor readings between .6V (decel) and 2.9V (heavy acceleration).

Advanced timing at idle within spec.

180,000 miles on my current timing belt.

Never had valves adjusted.

Performed voodoo by holding leaf at exhaust pipe. Leaf flops back & forth implying a leaky intake valve (at least that’s what I was told!). I’m going to test with a Cylinder Leakdown Tester.

Any other suggestions (besides the junkyard)?

The compression shouldn’t really be jumping around. If it starts out at 120-ish and then eventually pumps up to 160, your rings are probably pretty well worn (hardly surprising at 300k!). That may just be getting bad enough that it’s starting to misfire.

I would definitely give adjusting the valves a try, but I wouldn’t be hopeful. Also, if you do manage to get the thing running well again, you’re on borrowed time with that timing belt, so give that a change too!

Also 180k on your timing belt is way, way overdue for a change.

What are the mystery codes (DTCs) that it “pops”? What is the freeze frame data when a code “pops”? What is the STFT and LTFT? What are the MAF sensor valuse at different throttle positions and engine loads? Did you replace the catalytic converter based only on a DTC code, such as P0420? Are the values of the coolant temperature sensor, the air intake sensor, and the MAF sensor in line with engine speed and load, and with each other?

No mystery to the codes, they are all cylinder misfires (P0302, P0305, P0306, etc.)
I cleared the codes, connected a scan tool and cruised on the highway at 71 mph. Got the following readings.

LTFT=1.00, STFT=.97-1.04
MAP sensor = 15inHg
MAP sensor = 1.75v - 1.85v (.7v on decel and 2.6v on hard accel).
Replaced Cat Converter because popped code (P0420) long ago. Kept resetting and it would continue to come back.
IAT sensor=2.47V and 92degF.
O2 sensor #1 = .2v-.7v, O2 sensor #2 = .6v-.7v
Spark advance = 29 deg, 2500 rpm
ECT = 184 degF

Does the manual call for valve adjustment? If it has hydraulic lifters then it never needs adjusting. Does this car have variable valve timing?

If it’s adjustable lifters then I’m sure you’re way overdue. Tight valves will cause too much valve overlap, which has the most negative effect at idle.

This is at the end of the exhaust stroke and the beginning of the intake stroke. With both exhaust and intake valves are slightly open. At idle there is a strong vacuum behind the intake valve from the intake manifold and roughly atmospheric pressure behind the exhaust valve. So a little exhaust gas can get sucked back into the intake manifold before the exhaust valve closes completely. That gas dilutes the incoming fresh fuel/air and too much can behave much the same way as a leaking EGR valve.

Under load and at higher speeds conditions are very different and a little valve overlap can enhance performance. One benefit of variable valve timing.

So, the codes are set at idle, after the engine has warmed to 160F degrees and the engine control has gone into closed loop operation? The freeze frame data is set when the code sets, right? What is the freeze frame data? Is it of the right values?
I can’t but wonder if that worn timing belt (180,000 miles) may be allowing timing to slosh around so much that the spark can’t spark at the right times. You?
Have you checked the voltages on the sensors and actuators (like iac valve and egr) at idle? They could be off at idle, but within range at higher speeds or under load.
If you put a timing light on each spark plug wire (COP?) and observe the flashes (you won’t be checking timing), are they constant,or seem to miss?
How is the fuel pressure when the engine is misfiring? Low? High?

The shop manual does call for valve adjusting, which it’s never had. Your theory of tight valves sounds appropriate to my symptoms. I’m going to have the valves adjusted and timing belt replaced. It’ll probably be a few weeks before I can afford to have the car down for repair. I’ll let you know how it goes. Thanks for the advice.

Sorry for the late response, thought you’d like to know I just got around to getting the valves adjusted and timing belt replaced. Cost me $600 total and it runs like a champ!

Sorry for the late response, thought you’d like to know I just got around to getting the valves adjusted and timing belt replaced. Cost me $600 total and it runs like a champ!