Would removing your fuel cap extremely change your engines running ability.

[chasinthesun]

Ive replaced both fuel pumps ,replaced the tank and f filters but cant get the 87 190e to idle ,when I remove the gas cap and start the motor it sounds like its running on 3 cylinders ,put the cap back on and runs but no idle ,stalls out.Its a 4cylinder cosworth and has good compression , runs smooth at 1500 rpms and revs with no stumbles.

[optimusprime]

You cap must be on the tank and closed .It runs that way .You can check to see if the cap is good or bad by removeing it after a run .You should hear air being sucked back in to the tank. If you dont your cap wants replacing ..

[Frank Reiner]

chas:

Does the change in idle occur immediately with the replacement of the tank cap?, or is there a delay of a minute or two?

There is no functional reason for the tank to be closed for the engine to operate; the closure is for the purpose of controlling evaporative emissions. If the change in idle characteristics occurs immediately, there may be an air leak or stuck purge valve in the evaporative control system.

Typically, after operating for a period of time, the tank will exhibit a slight overpressure, rather than an underpressure.

[oldsinner111]

most systems I have seen,have tank gases going to a charcoal can.A vacumm line runs to can.And a larger line to intake manifold..These cans,and some have valves fail over the years.Try blocking off vac line to canister,and intake line.See how it runs.If better those components failed. You could just vent fumes to the air like old cars did.

[chasinthesun]

Their is a little check valve on the fuel line coming from the gas tank,could this be the suspect.In my original post I mentioned an Idle ,I mis spoke ,I have to keep her with half peddle to start ,then I can bring it down to 1500rpms ,any lower and it seems to start starving for fuel like a back pressure issue.