Looking into the combustion chamber

Blaise · Post by **Blaise** » Tue Apr 10, 2012 1:24 pm

Under certain conditions, I am getting a bit of pinging (detonation) at wide open throttle. I am doing a little homework to try to remedy this.

I would like to take a look into the combustion chamber to see if my piston tops show any sign of carbon build up.

Any tips on how I can take look in there?

I will keep you guys posted on what I find out.

Blaise

T.Hanson · Post by **T.Hanson** » Tue Apr 10, 2012 2:37 pm

Read the FAQ on timing. Google search the archives for previous posts on subjects of interest. Look for how to read your spark plugs on a web site for such things.

For the real time full color 3D HD view of the pistons and jugs you'll be learning how to remove the head on your engine. A very good adventure, but it often leads to unintended complications the first time, and unintended budget allowances, " While you've got it apart, may as well do this, that, this."

alotawatts · Post by **alotawatts** » Tue Apr 10, 2012 3:28 pm

What octane gas are you using ? Is it old ?

Blaise · Post by **Blaise** » Tue Apr 10, 2012 4:34 pm

T.Hanson wrote:Read the FAQ on timing. Google search the archives for previous posts on subjects of interest. Look for how to read your spark plugs on a web site for such things.

For the real time full color 3D HD view of the pistons and jugs you'll be learning how to remove the head on your engine. A very good adventure, but it often leads to unintended complications the first time, and unintended budget allowances, " While you've got it apart, may as well do this, that, this."

Oh, I don't plan on digging in to that degree. I just want to be sure that it is not due to carbon build up. I found a scope from Harbor Freight and will see if I can have a view in there.

Blaise · Post by **Blaise** » Tue Apr 10, 2012 4:36 pm

alotawatts wrote:What octane gas are you using ? Is it old ?

I use 91 octane, which is the highest commonly available in my area.

It is a 9.5:1 compression motor, running on motronic 1.3.

I took some voltage readings on the oxygen sensor, while on a recent trip.

This is just part of tuning the combination that I put in.

Keith · Post by **Keith** » Tue Apr 10, 2012 5:46 pm

A good way to knock the carbon off if you find some on there:
Get a large jug of water, and a piece of vacuum hose long enough to reach inside the cab. Find a downhill long enough and steep enough that you don't have to accelerate at all. While coasting, submerge the tube into the jug so that it starts sucking water. Do that until you get to the bottom of the hill. Compressing that small amount of water will knock a lot of the buildup loose without 'hydrolocking' the engine. To make sure you get everything, repeat the process a few times.
I've done this several times with my s38 before I rebuilt it, and also with my m30. It's worked like a charm on both of them, and solved all of my detonation issues. The s38 worked particularly well because of the vacuum ports on each one of the throttle bodies.
When I did it, though, neither cars had cats on them, so I can't attest as to whether or not this procedure would clog them up.

Blaise · Post by **Blaise** » Tue Apr 10, 2012 5:51 pm

Interesting. I had heard this before, but never tried it.

I kind of doubt that I have any carbon build up, as my motor has very few miles on it. But, I need to check before making the assumption.

Thanks for the tip

Keith wrote:A good way to knock the carbon off if you find some on there:
Get a large jug of water, and a piece of vacuum hose long enough to reach inside the cab. Find a downhill long enough and steep enough that you don't have to accelerate at all. While coasting, submerge the tube into the jug so that it starts sucking water. Do that until you get to the bottom of the hill. Compressing that small amount of water will knock a lot of the buildup loose without 'hydrolocking' the engine. To make sure you get everything, repeat the process a few times.
I've done this several times with my s38 before I rebuilt it, and also with my m30. It's worked like a charm on both of them, and solved all of my detonation issues. The s38 worked particularly well because of the vacuum ports on each one of the throttle bodies.
When I did it, though, neither cars had cats on them, so I can't attest as to whether or not this procedure would clog them up.

Keith · Post by **Keith** » Tue Apr 10, 2012 6:00 pm

No problem.
Carbon buildup can be a funny thing, though. It can come from soaking the pistons from running too richly. Bad valve stem seals are the most common culprit, though. I had an issue a few years back with a head I had rebuilt where they shop didn't properly install a few of the valve stem seals. I started getting noticeable oil consumption, and upon inspection (to my surprise/anger), the valve stems were soaked in oil.
If you're not getting serious consumption, though, I doubt that is the issue.

Blaise · Post by **Blaise** » Tue Apr 10, 2012 6:04 pm

Keith wrote:No problem.
Carbon buildup can be a funny thing, though. It can come from soaking the pistons from running too richly. Bad valve stem seals are the most common culprit, though. I had an issue a few years back with a head I had rebuilt where they shop didn't properly install a few of the valve stem seals. I started getting noticeable oil consumption, and upon inspection (to my surprise/anger), the valve stems were soaked in oil.
If you're not getting serious consumption, though, I doubt that is the issue.

I have about 4,000 miles since my last oil change (motor install) and have yet to add oil (I do check the level often). My fuel consumption is around 18mpg when I am having fun and as high as 24mpg on highway trips.

The car is a blast, but I have to be carefully at wide open throttle to back off if it pings.

Keith · Post by **Keith** » Tue Apr 10, 2012 6:11 pm

Yeah, that's a PITA. It sounds like a fuel supply issue. Do you notice that it occurs at full throttle at low engine speed, high speed, or both?
I'm inclined to think fuel pressure, or a clogged/malfunctioning injector(s). If it's happening when you go to full throttle even at high engine speeds, I would suspect fuel pressure first.

anthony · Post by **anthony** » Wed Apr 11, 2012 1:03 am

I have been running my m30b35 since mid January and I run the 91 octane. I am also in northern California. Only once did I inadvertently put in 87 and I got pinging.

But I am unable to go wide open throttle due to the fact I go from push rod to the bell crank and then convert to wire cable to throttle body. I plan to adjust so I do get wide open.

http://www.firstfives.org//faq/motronic ... ersion.htm

When I did notice the pinging it was on freeway onramp or uphill acceleration.

Falkenberg · Post by **Falkenberg** » Wed Apr 11, 2012 2:00 am

Be careful not to lose a part of the scope in the cylinder. It has happened.
The motronic you are using was designed for 10:1 compression?

Blaise · Post by **Blaise** » Wed Apr 11, 2012 12:29 pm

Keith wrote:Yeah, that's a PITA. It sounds like a fuel supply issue. Do you notice that it occurs at full throttle at low engine speed, high speed, or both?
I'm inclined to think fuel pressure, or a clogged/malfunctioning injector(s). If it's happening when you go to full throttle even at high engine speeds, I would suspect fuel pressure first.

The pinging, when it occurs, is on hard acceleration, at higher speeds. Getting on the highway, or going WOT from 60-70mph. If I back off a hair on the throttle, it still pulls great and the pinging stops.

Blaise · Post by **Blaise** » Wed Apr 11, 2012 12:32 pm

Falkenberg wrote:The motronic you are using was designed for 10:1 compression?

No, it was not. It came from a 1990 735i, which has an M30B35. I believe that this motor has a 9.0:1 compression.

I am running a 1985 euro M30B34, which runs 10.0:1 compression. That could be my issue, but either way, I have to solve it.

Keith · Post by **Keith** » Wed Apr 11, 2012 6:50 pm

You might consider a wide band o2 for diagnostic purposes. Innovate makes very affordable ones, and will be very helpful for your situation.