The Dutchman is heavily under pressure at Craven Cottage following a fifth consecutive defeat, with Sam Allardyce's team opening up a gap between itself and the relegation zone
Jol admitted in the build-up that a defeat to Fulham's London rival would leave him fearing for his job, having overseen the team's worst ever start to a Premier League season.
Modibo Maiga had the first half's best chance as he headed an effort against the post, but goalmouth action was infrequent for the rest of the period.
The hosts did have the lead less than 60 seconds after the break, however, as Fernando Amorebieta deflected a Mohamed Diame shot past Maarten Stekelenburg.
Carlton Cole and Joe Cole scored in the closing stages to seal an emphatic scoreline for West Ham, while Jol’s team suffered a fifth successive loss.
Sam Allardyce made four changes from last week's defeat to Chelsea, with Matthew Jarvis and Diame recalled.
Swansea City defeated Fulham last time out and Jol named three alterations to that team, as he dropped Bryan Ruiz and Derek Boateng, while captain Dimitar Berbatov missed out through illness.
The hosts dominated the early proceedings and nearly took the lead 10 minutes in as Modibo Maiga directed Mark Noble's right-wing free-kick goalwards, but it struck the post.
With just a few minutes of the first half remaining Kevin Nolan looked set to score as he met Downing's chipped cross with an acrobatic scissor-kick, but the attempt bounced up and into Stekelenburg's hands.
It took West Ham less than a minute to open the scoring after halftime, as Diame stole possession from Scott Parker and then saw his 20-yard effort deflected into the net off Amorebieta.
With just eight minutes remaining, Allardyce's men made sure of the win as Carlton Cole latched on to Downing's low cross and poked the ball home for his first goal since December 2012.
There was still enough time for Joe Cole to find the net in the 90th minute following an excellent Ravel Morrison run to heap more misery on Fulham and Jol.
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