348 sudden loss of power, rough after acceleration- ideas?

Hi folks. After letting the 348 sit for a couple weeks I took her out for a run to the coffee shop. After about 10 minutes of driving, I accelerated hard up to 4th, then slowed for the next light.

I could tell a change in the sound of the exhaust, and there is a noticeable loss of power. Slight shudder around 25 mph, but other than that, no indications of a problem. First thought is that one side wasn't firing, but no CELs.

Idle is a bit rougher, but still runs fine. Temps are normal, and engine starts right up. There may be a slight tapping sound on the passenger side, but it is hard to tell if I'm just being paranoid.

My first thought is that the belt skipped a tooth, considering it happened right after I had my foot on it, but I wanted to pick the brains of you engine experts so I can check anything else before dropping the motor to poke around. Seems like something is not timed right or not firing.

What else? No smoke out the back. Checked the cam phase sensor, and TPS are new. Also re-seated most of the electrical connections and plug wires. Nothing obvious.

Car is pretty well cared for and has always run like a top. Timing belt and bearings about 4 years, since I know someone will ask.

Thanks guys.

Any ideas?

 

Check igniters and coils. You can disconnect one at a time and see if you get a change. But first check to see that timing marks on cam gears are all visible at the same time through the little slots in the caps. Alternatively, take off the caps and inspect the cam belt.

 

You may have a catalytic converter coming apart inside, especially if you hear a rattle in one of the cats or see some white power type stuff in the exhaust tips.

 

What Dave said. You can buy a cheap engine stethescope at Harbor Freight. Get one (you can also use a screwdriver to your ear) and listen to the cats.

 

Roger. Thanks for the starters you two. I'll swing by HF next couple of days and get a scope; I've been needing one for a while anyway.

No unusual powder on exhaust when I checked, but will look more thoroughly.

Would it be helpful to have an audio clip of it running? I can record it next time if you think it would provide any insight.

Again, thanks.

 

After listening around the motor, there is definitely some noise on the 1-4 side (around 3 or 4), but I can't tell if it is a miss or a tap. It is not very loud. Almost sounds like an exhaust leak, but I can't tell for sure. I've always thought this motor was a bit noisy.

I any case, I pulled the timing belt cover off the 1-4 side.

Let me tell you-- if your timing cover nut decides to start spinning in the plastic base on the bottom half of the timing belt cover, you are going to have a bad morning. What a pain in the arse! Had to gently pry it out, but I'm happy to report that the cover is off and in tact.

Belt is definitely worn, but teeth look good. No cracks, oil fouling. Gates on the cam pulleys look good. I can see the timing marks on the cam pulleys, and they appear to be roughly in the same angular position. I'm guessing the other timing mark I'll have to pull the valve cover off to see? Plugz? Hate to burn a gasket but I will if I must.

Wires look good so I pulled the plugs. 1-3 are worn, and 4 appears carbon fouled and shorted (see pic)? I'm hoping I just had a misfire on cylinder 4 and the noise I hear is a little late-firing. All the other plugs had a good gap, only this one looked like that.

Off to enjoy my daughter's dance recital and pick up 8 plugs for installation tomorrow.

Will keep everyone posted, and if anyone has an picture requests while I'm in here, let me know.

Thanks again for all your help.
Image Unavailable, Please Login

 

Something isn't right in that hole for the plug to look like that. Metal in that hole. Bad news. Borescope time.

 

No metal shavings, just carbon build up-- the picture makes it look worse than it was. Definitely looks like something hasn't been firing right. Will keep you posted.

 

Dodged a bullet this time!

After doing a compression check and reassuring myself that the timing was not off, or at least not enough to cause any compression issues, I replaced all the plugs.

Started her up and she is purring like a kitten again! New plugs made a substantial difference on idle too.

I think what happened is that I added a bit too much oil on my last fill and upon my first hard acceleration, oil pressure toward the back of the engine increased and possibly fouled the plug. She never smoked, but it was definitely overfull by about 1/2 a quart.

Drained a bit of oil out to get it back in line, let her warm up and all noises are gone. The "tapping" I heard must have just been a misfire in #4.

In any case, thanks again for the help. Hopefully anyone else that has this issue can benefit from my experience: if it is running rough with no CEL, check the plugs and your oil level.