Engine lags

[AWDfrank]

Every so often my car will take forever to accelerate at normal or full throttle. The clutch isn's slipping at the time but it does have a cheap clutch in it. The car gets loud and boost high but will not accelerate at all, to the point of falling behind in traffic. As I said it only happens once in a while, other times it accelerates perfectly.

Please help me out, thankyou

 

[yokotabrat]

You could be getting knock. Whether it's real or not would take a bit more investigating, however when the ecu sees knock, it'll pull timing which usually results in a poorly accelerating car. The way to find out for sure would be to get a datalogger or hook up with someone that has one and log knock (assuming this is for the 90 in your profile).

There could be a number of other things besides knock as well, however since it's intermittent, it's what came to my mind first. Probably a good idea to do some basic maintenance first like plugs, wires, filter (air and fuel), etc if they haven't been done recently.

 

[AWDfrank]

Thanks man, I do need to do all of the things you said as in spark plugs and wires. Could my engine have been damaged as a result of this, or will the timing be pulled make it ok. Will doing all of the maintenance help the knock? Also will checking the compression of my engine be sufficient in knowing if it will withstand additional modding?

 

[andymoraitis]

Knock is caused by excessive combustion temps as a result of:

1. Hot air inlet temps
2. Too much timing advance
3. Insufficient octane
4. Too much boost for the fuel system or intercooler setup
5. Insufficient boost to kick the ECU onto the highest open loop timing map

It's basically a heat issue. Running a comp test will certainly tell you if the motor's in good health, but you'll need to follow the advice given above if you want to find the source of your knock without tossing tons of parts at the car. For all we know it could be a bad knock sensor. Get a datalogger, run a 3rd gear pull from 2500-7000 RPM and post it up so we can have a look.

Fixing general maintenance items likes plugs, wires and filters is always a great idea but it won't solve your issue if you're trying to run too much boost on stock injectors or if your ECU is trying to run 25 degrees of timing on high boost and pump gas with a small turbo. While a motor can become grenaded from knock, this is the exception rather than the rule.

Know what I mean?

Andy

 

[yokotabrat]

Edit: Oops, too slow on my reply. I'll keep it here as another voice to get a datalogger, though.

Since you're stock, the chances of you doing serious damage is fairly slim. The ecu pulls timing to help protect the engine and prevent knock from getting worse.

If you are actually getting knock, the regular maintenance could help with knock. It all depends on what's actually causing it. There are several things (off the top of my head) that can cause knock. The quality of the gas, intake temperatures being high, not enough fuel, improperly set base timing, excessive oil consumption. These are just a few of the common ones. The first step would be to verify that you are/aren't getting knock, though. No sense in troubleshooting something that may not be a problem. Dataloggers are relatively cheap if you already have a laptop or palm. Plus they are very useful when you start down the path of modifying your car.

Compression is always a good thing to know, but low compression is not necessarily a reason to not modify a car. I would be more concerned with how equal compression is across the motor, rather than a specific number. Unless that is of course you have something like 50psi in all cylinders, then I might be a little concerned.

 

[AWDfrank]

Thanks a lot.