Engine hesitates unless turbo spools up
 

I'm having this strange issue where my 951's engine runs as if it's misfiring at "city speeds", but then suddenly runs normally when the turbo spools up.

Any idea why it's doing this? My guess is there might be something wrong with the fuel system, but I don't know.

Thanks,

Most likely a vacuum leak between AFM and intake valves. Driving around in non-boost conditions, leak sucks in extra air that's not measured by AFM. As result, DME doesn't inject enough fuel and you get stumbling.

Under boost, the situation is reversed. Air that's measured by AFM is blown out of the leak by boost pressure. However, DME still injects sufficient fuel to match original air volume. Results in too much fuel, but this doesn't cause stumbling.

Common sources of leaks are:

- AFM clamp to J-boot
- J-boot clamp to turbo
- torn bypass valve disphragm
- short intercooler hoses, they get cut by too-tight hose-clamps
- conical hose going into throttle-body
- idle-valve hoses
- any tiny vacuum hose coming off intake-manifold
- intake manifold gaskets
- etc.

To fix, remove everything between AFM and intake valves and inspect for cracks and leak. Re-assemble with all new hoses and gaskets.

You can also remove maf sensor and use a Hardware store abs pipe cap or stock fuel filter works well to seal up on j-boot to put 10psi in there and find out

Thanks guys for the feedback.

I checked my engine and found no vacuum leaks. My next theory then was perhaps the ignition coil needed replacing, which I did but to no avail. After that, I consulted a mechanic who hypothesized a possible leak somewhere in the intake manifold, which I have yet to confirm.

Bad ignition would cause worse running under boost, not better. The mixture denser and more difficult to ignite.

hi,
sorry to hear that, i think maybe the ignition has some problem or the fuel pump broken.