My 99 Coupe (M6) has developed what is best described as a “subtle miss”. The symptom is that while driving at low-highway speeds (50-55 MPH), the engine is got a bit of a lopey note to it. It’s nothing as bad as an ignition miss (when one plug fail to fire), but real & noticeable.

Now there is some history here:

- The intake manifold was recently of so as to change an oil pressure sender. So one might be suspicious of intake leaks.

- The car did something very much like this once before last year, but only for a few minutes

So here is what I’ve done for trouble shooting so fat

1st experiment:

1.) Start at 800 RPM (in 5th gear). Open the throttle full. Load the brake so as to hold RPM fixed.

- Result; Perfectly uniform growl/buzz

2.) Modulate the brake to allow RPM to climb slowly so as to investigate operation (still full throttle) at higher RPM.

- Result; The exhaust note turns "lopey" as the engine goes past 1100 or 1200 RPM. It is not as severe as what you would get from full loss of a cylinder (as in a fouled plug), but real & noticeable. Above about 1500 RPM it is not noticeable (but this may be due to my inability to detect at this higher frequency). I only had the opportunity to do this once, but I can probably repeat it soon.

2nd experiment:

At idle, pull one injector connector at a time & note change in exhaust note. Result; They all sound identical & very dramatic (exhaust note changes to a pop-pop-pop-pop-pop....)

Details on the previous “event”- This car had a one-time event noticed at highway speed last year where it had what seemed like a miss, meaning un-even power delivery like above. It persisted for a couple of miles then just faded away. There was nothing dramatic about how the symptom disappeared (dramatic meaning like a plug which starts to fire intermittently & then recovers). There was just a smooth disappearance of the symptom.

More info: I checked for a manifold leak by spraying propane around the intake-to-head joint while at idle. No effect. I also got a hose & listened/probed along the head-manifold joint listing for the whistle of a leak. None found.

Conclusions;

- I'm now thinking it is not a leak at the manifold (because I can't detect a leak & the symptom shows up at wide-open-throttle where manifold vacuum is very low & leaks become less significant).

- I think I can rule out loss of compression since sub-1000 RPM is fine, & that is actually where loss of compression is most noticed.

- How about an intake or exhaust valve that hangs open at super-1000 RPM? .... I think I'd hear the change in valve lash racket. Not likely.

- One suspicion is a defective injector. I need to invoke something that functions OK at low pulsing-rate (below 1000 RPM) but fails to deliver fully at higher pulsing-rate. In addition it needs to be an intermittent failure such that it caused last years symptom, then went away.

So here is my next troubleshooting plan

-Do a full-throttle, fixed RPM pull (loaded by the brake) at 1200 RPM & then a plug chop. Inspect all plugs for the odd-ball. If I find that odd plug, then I'm positioned to swap cylinder-specific hardware around to try to isolate the cause (plug, wire, coil, injector).

-I also have the option to hook up an OBD-2 computer. I might see just how the controls are reacting to what ever this malfunction it. That could be a big hint.

Thoughts?
Mike

p.s. No, it doesn't have a check-engine light on

 

What gear are you in while you are doing 50 mph?....sounds like you are laboring the motor perhaps in 6th gear. If you do 50 mph in 5th or 4th does it do it? Try increasing your mph or switching to a higher gear (4th/5th. I would think that if it was a random misfire the computer would pick it up........just my $.02

 

I first noticed the symptom at about 55 in 6th gear. I know that changing gears makes the symptom go away, but I want to know what is causing this. IT'S NOT SUPPOSE TO DO THIS & IT DIDN'T USED TO DO IT.

 

Just because the CEL is off doesn't mean that the PCM hasn't seen the problem. First thing I'd do is check and see what DTC are set in any/all the car's computers.

Second, is the problem related to engine speed or road speed (if it does it at 1500 RPM in 6th - does it also occur at 1500 RPM in 4th and 3rd .... or is gear selection immaterial - it occurs at 55MPH) ??

 

I had similar symptoms several years ago. Ended being a plug wire that was not completely seated on the coil.
When I found it I pulled the wire, to check it, and found both the wire and coil had green oxidation from the poor connection.

Good luck.

 

Good point. I hav access to an OBD-2 computer. I'll hook it up & see what is in there.

The problem is independent of road speed/gearing. It only depends on RPM & throttle setting.

Mike

 

You don't need a computer ... the C5 has its own diagnostic that will show you the codes from every computer in the car. Most OBD2 computers can only read the codes from the PCM.

Here are instructions for entering the display mode ......

https://www.corvetteforum.com/techti...D=26&TopicID=1

Pretty neat ..... give it a try .....

 

Stephen,

Thanks much, I'll try tonight. I had not idea that this capability was built in, but this fits into the category were we've often asked..... "wouldn't it make sense if GM provided this particular tool for me. Just how hard can it be? It's just a little software."

In this case, GM did something very logical & effective!

Mike

 

So I did the plug chop test. The lope was pretty pronounced when I did this. I had a good 30 second pull at 1500 rpm, killed the engine & coasted to a stop.

What I got was 8 nearly identical looking plugs. This was not what I was expecting.

Mike

 

This readout works great. What I did find was no engine codes at all, so I have no hints. But it is a very cool feature!

The one thing I have excluded is plug wires. I broke a couple getting them off, so I fingured 9 years is good enough mileage to get out of set. I replace them. Symptom is still there.

Strage thing is, the severity of this changes day to day. Sometimes it is so subtle I could be convinced it's all in my head, other days it's pretty dramatic.

Mike

 

It is so logical and effective that GM has DELETED this feature on the C6 Corvette. Who knows why ... dealers complaining that they were losing service business ??? Maybe GM thought we were getting "too much" information ..... regardless, this function is gone on the C6.

 

It may not be an engine code ..... for example (AND THIS IS ONLY A POSSIBILITY) the EBCM is able to ask the engine to deliver lower engine torque during a Traction Control "event". If the EBCM is getting flaky (which MIGHT cause it to post a code) ... maybe you are seeing the EBCM asking for reduced engine torque for a moment (which changes engine timing) then allowing full torque ... then reducing it again ....

So, clear ALL codes in ALL computers and drive the car for a day or two. Then see what, if any codes post in ANY computer and list them in this thread.

 

Cleared them all today.

 

So here is the final status (I think).....
I hooked up an OBD-2 computer to monitor the fuel trim in real time figuring that if it's an injector, an indication is likely to show on the affected bank.

It works great. We can watch the trim make tiny adjustments... BUT; the problem has drifted off into obscurity again. It's gone for now.

I continue to be suspicious of an intermittent injector, so there is a can of Techron in the tank & I'll run a second through it.

 

I had a problem that sounds identical to yours,I also did checks similar to yours,and after all the checks nada, nothing conclusive.I sat and thought about what was happening and remembered a problem I had with a fast street car many years ago, it seems I had a weak coil, it would fire the plugs and felt ok but if there was a condition that required a real good spark like a part throttle cruise condition where the mixture gets lean its noticeable, funny thing the car seemed to run fine at WOT after I changed the coil the car pulled stronger in high gear.You may have one coil that is weak (not failed) it fires but W/aweak spark & not have complete combustion in that particular cylinder under certain conditions. You can try to isolate it by wetting each coil w/a spray bottle if the problem is internal you can get a known good coil (thats the term Chevrolet used) (they mean new)and switch it around until you'v e tried every cylinder or if you have a real good friend W/a vette he'll let you try all his coils at once Anyway thats my $.02 worth I hope my advice helps.