15psi boost = blown injector fuses

-nick · Post by **-nick** » Tue Aug 29, 2006 8:12 am

I'm not sure which section to put this in, but here it is. I've been tuning / playing around at about 11psi boost pressure. Last night I turned it up to 15-16psi for the first time. It was running well doing some short ~70% throttle spurts. Then I gave it a good full throttle shot for a couple seconds, let off, then again. The second time I hit the full 15psi boost the car suddenly stalled completely.

I discovered that both of the 5amp injector fuses in the relay board had blown. Any ideas why this happened? Unfortunately, I wasn't data logging at the time (of course...). I thought someone might have some thoughts?

MS-II, v2.2 main board, relay board, EDIS, 38lb injectors @ 3bar, 2.0L turbocharged four cylinder, wideband (no correction at the moment), v2.36 megatune

EDIT - here's an attachment of just running around the block. Maybe something sticks out? Other than being very rich everywhere (just started tuning!)

jakobsladderz · Post by **jakobsladderz** » Tue Aug 29, 2006 6:55 pm

Short of further information I would guess that the injectors you are running are low impedance and that you have not got any PWM or resistors set up with them. At low duty cycle (low boost/off boost) the average current (which blows the fuse) would be less than 2.5A per injector. Once the duty cycle increases enough (as it would running 15PSI on 39lb injectors, maybe 85-90%) the average current would also increase to the point where they'll blow. Probably better the fuse than the injectors though. Either put resistors in series with the injectors (see the manual for advice on size and rating) or look at the flyback board and PWM.

Duncan

-nick · Post by **-nick** » Wed Aug 30, 2006 7:54 am

Hi Duncan,
Thanks for the feedback. From my latest run around the block, I got an 72% duty cycle at 196kpa, 5073rpm, 72% throttle, and 10.8:1 AFR. So yeah, you're right about maxing out the injectors! Even after leaning things out, it looks like I need to step up the injector output.

However, I'm using standard issue Bosch "brown top" injectors (from a Merkur, among others) which should be regular high-impedance.

Could the fuses have blown from simply pushing the duty cycle too high?

Looks like injector-shopping time regardless

BottleFed70 · Post by **BottleFed70** » Wed Aug 30, 2006 1:55 pm

Fuses dont blow instantly.. What happens with something like fuel injectors(on-off very quickly) is that the fuses start to act like a fuse for the average current...not the peak current. When you increase the duty cycle, you increase the average current through the fuses which can cause them to blow. I would step up to 7.5amp fuses....

-nick · Post by **-nick** » Wed Aug 30, 2006 1:56 pm

just checked the ohms across the injectors and they're low impedance after all. Yikes. Could I have hurt anything while running them??

The blown fuse mystery is now solved.

jakobsladderz · Post by **jakobsladderz** » Wed Aug 30, 2006 2:23 pm

The injectors would have been protected by the fuse which is good, so they'll be fine. I'd say the driver transistors should be OK, I'd say they're rated for the current and as it wasn't running for too long they won't have overheated. Get yourself some resistors for the injectors for a start (or look into the flyback/pwm feature) and adjust the boost to limit the injectors to the right value. Tune it till you have it sorted at the low boost and then maybe look at getting some bigger injectors and winding up the boost more. 16PSI can cook things fast, best to stick to 12-14 then be a few HP down and 1 engine up.

-nick · Post by **-nick** » Thu Aug 31, 2006 7:53 am

According to my peak kpa, I'm only seeing 13.6 psi boost. Either my VDO mechanical boost gauge is off, or my brain isn't processing the quick glance over to it as the road is rushing up.

I'm trying to get a set of stuck rings to seat (new motor was assembled then sat for a year +). Nothing has worked so far, so I thought I might hammer on it a bit before conceding and pulling the pistons out for a re-ring.

Rather than messing with the low impedance injectors, which ultimately can't supply enough fuel, I'm looking for high impedance replacements at a slightly higher delivery rating (started another thread in the injectors/fuel/etc. forum).

Thanks all for the help, it's been invaluable as usual!