Single Knock Sound when the gas is initially pressed

[Bluey]

Hi,

I think I posted referencing this problem before, but could not find my old post.

This problem has me worried and I'm hoping someone can suggestion a next step:
My PT is a 5 speed. Here's an example of what happens (it actually happens in any gear except the sound is not as loud in the 4th and 5th gears). Say I'm driving in slow traffic in 2nd gear and ease up or release the gas pedal to slow down. If I stay in the gear and press the gas again, I get a single knock…happens almost everytime. If I rev high, then I don't get the knock. This goes for any gear. I get the same sound after changing gears and pressing the gas again.

Any ideas what this could be? On 3 out of 4 visits, the car came back "no fault found".

Here's some history on when all this started.
Earlier this year, one day on the way to work, my car started jerking and hesitating. It turned out that the plug gaps were quite big and the dealership replaced the plugs and wires. After picking up the car, I noticed that the engine was not running so smooth anymore, could feel a very slight stumble at idle. Thinking it was a bad plug (or incorrect gap), I took it back to the dealership. They replaced the engine mounts claiming that they were cracked. This did not fix the stumble problem, but a new problem came up…the knocking sound I described above. On my 3rd visit, per some suggestions from my old post, I requested the service manager test drive the car with me present. He heard the sound I was talking about. Also per a suggestion posted, I ask that he make sure the engine and tranny were aligned properly after the engine mounts were replaced…all checked out okay, but they said that the lower control arm bushings were torn , and that this could cause a knocking type of sound. They replaced the lower control arms under warranty, but the problem still exists.

All of the above (except for the spark plugs) were covered under warranty and were done at the same dealership. I am concerned that if I let this problem go, it is just going to get worse, and I am currently at 67,000 km on the odometer…30k more, and I'm out of warranty. With my luck, if I don't get this fixed asap, it will cost me major bucks the day after the warranty expires.

Any ideas what this problem could be or how I should proceed?

Thanks in advance for your help.

Bluey

[2 TONE PT]

Still happenning and no cure yet? I've still go the same problem and no luck identifying the source by my dealer either. The lead tech at the dealer managed to continuously reproduce it but couldn't locate it. The conclusion was that is was just a normal sound inherent to the PT when the driveline gets initial loading.

Let me know if you have any luck with it.

Steve

[GOLDMEMBER]

Does it make a difference if you change brands of gas or Premium instead of Regular..

I heard that the Gasoline additive MMT and PT Cruisers don't mix from a DC source. I heard that Esso doesn't use MMT in Ontario. Its worth a try.

Another thing to try is having the injectors cleaned or run the injector cleaner throught the cruiser

Cheers

[2 TONE PT]

Originally Posted by GOLDMEMBER

Does it make a difference if you change brands of gas or Premium instead of Regular..

I heard that the Gasoline additive MMT and PT Cruisers don't mix from a DC source. I heard that Esso doesn't use MMT in Ontario. Its worth a try.

Another thing to try is having the injectors cleaned or run the injector cleaner throught the cruiser

Cheers

The knock isn't an engine knock it's more like a mechanical clunk. It sounds to me like slop in the driveline or mechanical interferance when first seeing load.

All I use is Esso glad to hear they don't use the MMT additive.

Steve

[Sci-Fi]

Have they checked/replaced the transmission mount? If they replaced the top and bottom motor mounts, I would think they would also replace the transmission mount.

[Jobi Wan Kenobi]

Wow, this is weird. I have a 2002 with 27,000+ miles, and have experienced the same things. If I'm driving through town (in other words, not at fast highway speeds) and let up on the gas, then press it again, I can hear a clunk up front. I haven't pursued this with the dealership. Honestly, I figured they would think me crazy.

I have also experienced the jerking/hesitating. My engine had started to run real rough. I took it in and they replaced the spark plugs. This was done on Monday, and I thought it fixed the problem. It ran find Monday evening and Tuesday morning. However, when I left work yesterday and pulled into the gas station, it started up again. Part of my problem is that the engine doens't idle properly. In Park, it can fluctuate between about 800 and 1400. It never did this in its first year and a half, so I don't think it is right. I also think that this rpm fluctuation may be the cause of the hitching/jerking sometimes while driving. Mine happens more when I am slowing down. When I come to a stop it acts like it doesn't want to shift down, as the engine hasn't slowed down enough.

I'll keep an eye on it over the long weekend. If it continues, I'll use my digital camera which can take short movies and record the Tach doing its little dance. This may sound silly, but the first time I went to the dealership they took it out on the road and claimed it ran fine for them. I wasn't suprised, as it doesn't misbehave 100% of the time.

Well, enough rambling. I'll be interested to hear if your clunk is diagnosed. If so, mabye I'll go in and ask them to check whatever they find on yours.

[Rolo]

They both (Jobi, Bluey) sound like classic fuel/fuel delivery problems, especially since your plugs were not firing properly for a while; you likely have buildup.

- Get the injectors & TB cleaned. You may need more work than what fuel additives can do. Use this as an excuse to get bigger injectors, hehe.

- Replace fuel filter. Check your entire fuel line for kinks, pinches, etc.

- Which octane gas are you using? If you aren't using premium (91 or better), then try it for a few tanks to see if the knock subsides; that will help isolate the trouble.

- For troubleshooting, keep your tank 3/4 full or more in case the fuel pump is being an ice-queen and not putting out as much as it should.
-- Make sure your air intake and filter are clean, too.

I would be very surprised if it were a balancing/mount problem.

If fuel delivery is definitively ruled out, then start looking at sensors giving the computer incorrect information.

Let us know what you find!

[Bluey]

Originally Posted by Rolo

They both (Jobi, Bluey) sound like classic fuel/fuel delivery problems, especially since your plugs were not firing properly for a while; you likely have buildup.

- Get the injectors & TB cleaned. You may need more work than what fuel additives can do. Use this as an excuse to get bigger injectors, hehe.

- Replace fuel filter. Check your entire fuel line for kinks, pinches, etc.

- Which octane gas are you using? If you aren't using premium (91 or better), then try it for a few tanks to see if the knock subsides; that will help isolate the trouble.

- For troubleshooting, keep your tank 3/4 full or more in case the fuel pump is being an ice-queen and not putting out as much as it should.
-- Make sure your air intake and filter are clean, too.

I would be very surprised if it were a balancing/mount problem.

If fuel delivery is definitively ruled out, then start looking at sensors giving the computer incorrect information.

Let us know what you find!

Thanks All for your input. Steve, I didn't realize you had a same problem.

Per Steve's message, the clunk doesn't seem to be related to the engine/fuel. It's more mechanical and sounds like something hitting when the initial load is applied to transmission by the engine (imagine the sound of throwing a small stone at a piece of cardboard). I will have the injector cleaning service done as I know it is about time to do it anyway.

Also as GOLDMEMBER suggested, I will try ESSO and see if it makes a difference. I currently use Shell as it seems to be giving me the best mileage of all the major brands.

Thanks again everyone.

Bluey

[2 TONE PT]

Jobi,

I had a similar problem with fluctuating RPMs. It never triggered a check engine light. It only did it some times and of course never at the dealer. I finally got it to the dealer and left it running while I got the tech to come out for a drive. He put on the scanner that showed the throttle body sensor giving a signal variation up to 3%. They replaced the throttle body sensor and of course no more problem for a few days. It went back in and they kept it for a day to reproduce the problem again (fortunately thats all it took). The end result was that there was a small break in the signal wire from the throttle body sensor to the computer, it worked most of the time but could be opened by stress (torque maybe) or temperature variations. They put in a jumper wire to replace it and no problems since then. Hopefully that'll do the trick for you.

Steve

[Richard in Idaho]

I had fluctuating RPM in my last car. It did not cause the check engine light to come on. Dealer could not fine the problem. After months of this it set the check engine light. Turned out to be the EGR valve. Was told latter the the EGR is the first thing to check with fluctuating RPM.