hi guys,

I've got a D15 engine on a eg3. The engine seems to be backfiring into the throttle body once it revs up. It produces a fluttering noice above 2000rpm. I'm quite certain it's trying to force air or exhaust fumes out of the throttle body.

I've checked the cambelts, ignition timing, all fine. The valvetrain is also in good condition. However, compression test yielded 190psi for piston 2, 3 and 4. But piston 1 is only 160psi.

No traces of blown headgasket. No smoke from the exhaust either.

Could it be problem with a bent valve thats why its not sealing up piston 1? What can me the problem?

Any help will be great!

Thanks!

 

wow. seems weird to me. my first guess woulda been a timing issue. but u said that checked out fine. so my only other guess would be a intake valve not sealing (as u mentioned) thats the only other way i would imagine that air could be forced back. unless that isnt what it is doing exactly then u will have a whole other set of options to check. hope u find out what it is. but yeah id personally check for a burnt or bent valve.

 

If you had bent valves, it would have way worse compression readings. And since mechanical and electrical timing check out, I would wonder if it's getting enough spark as you rev it. Maybe it gets enough at idle, but not enough at speed. Have you pulled the plug and checked under the cap for any culprits yet? Could also be a clogged injector spraying lean causing that hole to detonate a little. Check the electrical stuff first.

 

i've opened up the dizzy cap and give it a good clean. I was worried the fuel filter was blocked before, but it's pumping out alright. This engine is a single point injection so it shouldn't be just affecting piston 1. It may be fault with the ignition. But what has caused the 30psi loss in compression in piston 1? Maybe the valve is only slightly damaged and not seating properly. I'll be doing a leakdown test tonight and see what's happening.

 

Ahhhh, right. SPFI. Anything could be causing that compression problem. Even carbon build-up on the valves can bang the hell out of the seats causing what you are describing. If you're really worried, leak it down to see where the compression is going.

Does it burn oil or anything?