Lumpy Idle upon start

D

djd234

Member
Hi all. On my z4 e86 3.0si that I’ve just gotten about a month ago I’ve been having a strange lumpy idle upon starting both hot and cold that then smooths out after about 30-60 seconds. I wouldn’t say it’s very rough only slight but still unusual. Also tried starting with no MAF and becomes smooth. Is it conclusive to suspect MAF is bad? Also no codes being thrown to Carly.
 
Check your fuel trims (4 of them) to see if the car is running lean. If it is, then smoke test it to find the vacuum leaks.
 
Your screen shot shows the long term fuel trims but not the short term fuel trims.

Some vacuum leaks are small enough for the engine DME to manage when in closed loop control (when the engine is hot). They cause misfires when the engine DME is in open loop control (when the engine is cold).

So, we are trying to see if you have evidence of a vacuum leak.

Try this:
With a hot engine at idle, use your scanner to look at the short term fuel trims (2 of). Note the numbers.
Now rev the engine to say 2,500 rpm and watch the short term fuel trims.
- If they reduce significantly, might even go negative, then you have a vacuum leak to find.
- If they don't move significantly, then you don't have a vacuum leak.

Unplugging the MAF makes the fuel mixture very rich, This can mask a small vacuum leak.
 
NZ00Z3 said:
Your screen shot shows the long term fuel trims but not the short term fuel trims.

Some vacuum leaks are small enough for the engine DME to manage when in closed loop control (when the engine is hot). They cause misfires when the engine DME is in open loop control (when the engine is cold).

So, we are trying to see if you have evidence of a vacuum leak.

Try this:
With a hot engine at idle, use your scanner to look at the short term fuel trims (2 of). Note the numbers.
Now rev the engine to say 2,500 rpm and watch the short term fuel trims.
- If they reduce significantly, might even go negative, then you have a vacuum leak to find.
- If they don't move significantly, then you don't have a vacuum leak.

Unplugging the MAF makes the fuel mixture very rich, This can mask a small vacuum leak.
Click to expand...
Okay il try this. Thanks. If I remember correctly when I pressed on throttle the values for AFR did change for split second then corrected back so that may hint. It’s all quite unusual and the car by no means drives ‘bad’ but I’m sure there’s something up
 
How is your valve cover gasket just out of interest? Just had mine changed today and the idle is now pebble smooth. Mine was leaky.
 
You must log in or register to reply here.
Back
Top Bottom