1998 Suzuki df 70 (Evinrude) - Surge @ 3,900 rpm

Symptoms: (1) Starts and runs perfectly up to 3,900 rpm. (2) At 3,900 rpm when throttle advanced, surges between 3,900 and 4,200-4,400 rpm at approximately 1-2 cycles per second. (3) At WOT surge stops but max rpm 3,900.

Change/Tested: (1) Throwing no codes at any time. (2) Run on portable tank with new fresh gas. (3) Plugs (twice). (4) Complete lp and hp fuel system check/disassmbly (hp pressure exactly on spec and rock solid through entire rpm range).

(5). Beginning to suspect MAP sensor. Checked voltage per service manual. At 0 kPa right on the money @ 4v. At 40 kPa also correct @ 2.4 v. But cannot get to 80 kPa with hand pump. And MAP sensor isolated will not hold vacuum. Takes 3-4 seconds to bleed off vacuum when pumping stops. Hose checks out fine - no leaks.

Question: Should MAP sensor hold vacuum? (Don't know the internal mechanics but acts like there could be a diaphram that has started to leak).

What else could be the problem?

Re: 1998 Suzuki df 70 (Evinrude) - Surge @ 3,900 rpm

Have you had the injectors cleaned and flow checked?

Have you checked the timing at 3900RPM?

Re: 1998 Suzuki df 70 (Evinrude) - Surge @ 3,900 rpm

lakensea said:

Have you had the injectors cleaned and flow checked?

Have you checked the timing at 3900RPM?

Click to expand...

I hecked each individual injector/plug as the source of the problem by disconnecting each injector one at a time in sequence. The surging occurs in each of the 4 cases which leads me to believe that I have a system problem and not an individual cylinder (plug/injector) problem.

I have not checked timing but will do so as soon as I can.

Any thoughts on something that would affect all cylinders at once, like a MAP sensor?

Re: 1998 Suzuki df 70 (Evinrude) - Surge @ 3,900 rpm

lakensea said:

Have you had the injectors cleaned and flow checked?

Have you checked the timing at 3900RPM?

Click to expand...

Timing @ beginning of surge is 22-23 deg BTDC under full load.

Haven't had injectors cleaned/checked yet. What is the chance that all 4 are clogged at the same time?

Beginning to think ECU.

Any other ideas?