No High RPM

[daveinrenton]

Don and friends:

After years of flawless performance, my A4 is doing something I can't figure out. The engine starts right away and idles smoothly. But when I put anything more than a little gas to it, it chugs and starts up this knocking sound that sounds like a diesel idling, kind of a rolling/knocking sound. Full throttle and it chokes on itself completely and will die if not throttled down. Any ideas on what could be causing it? Bad gas, bad air, something failing internally? I am hoping since it idles OK that there is something with the fuel-air that is causing this but the knocking made it sound like something was a mess in there. I changed the spark-plugs for new just for yucks and there was zero change in performance. Look forward to some possible causes.

Thanks, Dave

[ndutton]

My first thought (only a guess) goes to exhaust restriction.

[edwardc]

That was mine too. An internally delaminated exhaust hose can get restricted abruptly with no external evidence.

[ndutton]

That was mine too. An internally delaminated exhaust hose can get restricted abruptly with no external evidence.

That and the mixing Tee are the usual places.

[daveinrenton]

Here's a little more. The exhaust hose is new, just few years old and my old one did collapse inside but did not cause this symptom. Here are couple more details I failed to mention. When testing the engine, my cabin carbon monoxide alarm went off. This has never happened before so gasses were coming in straight from the engine. Also, the pressure of the water exit with the exhaust visibly reduced like there was less exhaust mixing with it. The volume was about the same, the water was just less chaotic coming out plus a jetting noise was happening out the tail-pipe. Talking with an car engine man friend of mine, they said it might be a blown head gasket. The CO leaking into the cabin made me think this might be it. I need to check the perimeter of the motor to see if there are signs of that. Does this stimulate some new ideas?

Dave

[tenders]

This reinforces exhaust blockage to me.

[Dave Neptune]

Bettin' exhaust here too

Dave, I just went through the same scenario. I to agree it is the "hot section" of the exhaust. Once it starts restricting to the point of affecting performance the degradation from that point on is quite fast!!!!! Mine actually blew the exhaust gasket and was leaking exhaust fumes into the cabin. I doubt it is the head at all.

Fabricating a nes section is actually not that bad of a job.

Dave Neptune

[ndutton]

Talking with a car engine man friend of mine, they said it might be a blown head gasket.

Easy to test before you haul things apart with a compression test. This reminds me of another one of my life experience stories.

My Dad fancied himself a mechanic. He took care of our family car pretty well but was one of those guys who always had a part or two left over on the bench. Looking back though, his diagnosis skills were questionable if not downright comical:

Engine missing or otherwise running poorly = cracked block.
Hard to start = cracked block.
Smoke out the tailpipe = cracked block.
Smoke out the crankcase breather = cracked block.
Fouled spark plugs = cracked block.
Overheating = cracked block.

Oh yeah, there was one departure from the trend,
Excessive pinging under load = perfect timing.

[tenders]

That reminds me of the old "Far Side" cartoon:

http://25.media.tumblr.com/tumblr_lflx76mIJe1qcmrkno1_500.jpg

[Mo]

I'm with compression test

Compression check is in order for sure. Might be something as simple as valves when it idles fine and dies on throttle-up. If the exhaust was worked on were there any changes made that could have allowed water to get back into the top end adversely affecting your valve performance.

I've worked on two in the past year or so with stuck valves. The engine ran fine at idle and would also throttle up at idle. When put in gear forward it would stumble out and stall or near stall ... and wouldn't throttle up. Basically it didn't have enough compression due to valve leakage and ... just didn't have it. Reverse was a little better due to the ratio change but again was nowhere near performance level.

So, I'd like to suggest:
-compression test with a gauge. The thumb method didn't work on the first engine I worked on (it checked out) and I overlooked the problem, went looking elsewhere only to find the compression was shot on 3 and 4. That was my learning curve.

-also check your electrical system, plugs all working, wires good etc.

[Skywalker]

Tenders

I am a huge Far Side fan.

Thanks for the post!

[daveinrenton]

Great stuff, guys. I am sure hoping it's an exhaust blockage. That's a lot easier to fix! I'll do the compression test with a gauge since that's also easy to do. If that checks out, then I'll check the replaced exhaust hose then the hot section. Your comments are always appreciated and the Gary Larson material, too!

[67c&ccorv]

Great stuff, guys. I am sure hoping it's an exhaust blockage. That's a lot easier to fix! I'll do the compression test with a gauge since that's also easy to do. If that checks out, then I'll check the replaced exhaust hose then the hot section. Your comments are always appreciated and the Gary Larson material, too!

I take it you never replaced the hot section with the hose?

[daveinrenton]

No, I never replaced the hot section ever. It was the hose to the stand-pipe muffler that took the cooled exhaust out the hind-end. I just got back from a biz trip so I will inspect the hose I replaced first and see if it did not cave in on itself. I took that off the list because it was pretty new at just a few years old. But, who knows? If the occlusion is there, it would cause back-up into the manifold and cause symptoms including forcing gas out a micro-crevices. The clog might be in the hot section which I will check next. Could be rust blockage as I have never had it off to look inside in the 12 years I have had it. I have not done the compression test yet and so will get around to that, too, just to make sure. But, if I find the exhaust blocked, that is most likely it and I will report back to you all. I'm looking forward to sweet, smooth running again!

[roadnsky]

Might want to read this from Don...

http://www.moyermarine.com/exhaust.htm