I have a 1990 Hymer on a merc 410 chassis, 2.9 om602.
It’s a large Hymer conversion, 660s, 6 berth and I also tow a small car.

The engine has been turbo intercooled, by JB intercoolers a few years ago, by another owner. Engine has done about 100k miles and is a good runner, starts first time, no smoke etc. My problem is lake of power. On a recent holiday to Italy, I had to drive it flat out all the time to keep up to 50 to 60mph, which rapidly dropped off to 25 to 40 on slight inclines. I also notice a huge change in power with different weather conditions (I think high and low whether pressure can affect diesel engines, but I didn’t realise to the extent it does, about 20 mph!) I have tried all sorts, fuel filters, checked for blockages, been back to the turbo garage, who checked waste gate etc. It seems from what I gather on the net that this engine in NA form is 95bhp and obviously higher with the turbo. But also on the web, it seems that the om602 in other variations ranges from 125 to 300 plus bhp tuned?

My question is, is this normal speeds to be getting from this engine? If so, is there any way I can substantially improve this? Tuning or complete engine swap for something else that will drop in without too much modification? I love the Hymer and don’t mind spending some on the engine to speed it up rather than buying another camper.


I want to be able to cruise at 60 without being flat out all the time, and for it to maintain most of that speed on moderate motorway inclines. I was often in 2nd or 3rd gear on the motorway in France and I’m not talking about going over the Alpes, just normal flattish roads.

I can’t seem to get to bottom of weather this engine is under powered or not. As its getting on a bit, nobody seems able to tell me. I get conflicting options from other owners, some say the 2.9 engine in these merc Hymers is fine, others say it is very slow. I guess I need to stick it on a rolling road to see what BHP it’s chucking out, anybody know of one in the West Country that will take a truck?

Unless anyone comes up with something amazing, I’m kinda fed up with it now, and am swaying towards an engine swap with something reliable ,but much more powerful /torky.

Does anybody have any experience with what will fit/bolt in without chopping bodywork around? Or can the existing om602 be rebuild/rebored, larger turbo/intercooler etc. etc. to produce a meaningful increase? Or better still, does anybody know who specializes in tuning these engines, preferably in the UK, but not essential?

I am determined to sort something out ready for next summer, I’m fed up with trying to overtake artic lorry’s doing 56mph, on the motorways, getting halfway past, hitting the tiniest of inclines, and then slowly dropping back, with my foot to the floor!

Any help advice, much appreciated.

John