1999 200 EFI Erratic ECU values

**lost2a6** · 02-23-2018, 06:37 PM

I'm working on a 1999 200 EFI Merc that has very erratic ecu values when running. RPM, TPS, Baro press, coolant temp, inj on time, pretty much everything is jumping all over the place. I'm using the Rinda Technologies Diacom software. I've swapped ECU's, switch boxes, removed the idle and knock modules. I pulled the flywheel and checked the splines. I've verified 12v, key on 12v, and a ground at the plug going to the ECU. I've replaced the plugs with a resistor type. All six plugs have a good strong fire. It's like the ECU momentary looses power or a ground. The boat seems to run ok other than missing some RPM on top. The only thing that I'm wondering is if the plug is actually making a good connection. I've ran out of ideas at this point. This is a fishing boat and has seen it's share of salt water. Thanks!

**tux974** · 02-24-2018, 06:23 AM

Some electrical gremlins can be a real pain to locate. Try a remote key harness also try to move your harness pin connector while it is running.
I have also seen ECUs act up once they heat up. You may sometimes even fix a problem by moving things and never know what it was exactly.

**tlwjkw** · 02-24-2018, 06:41 AM

if tha ecu (or anything 'lectric) is screwed up it ain't gonna do this...............

Originally Posted by lost2a6

The boat seems to run ok other than missing some RPM on top.

**billybob** · 02-25-2018, 09:43 AM

Merc. tech. says, when you have erratic values. always check & clean all grounds first. saltwater motors usually have corroded grounds. creating high resistance.

**W2F a V-King** · 02-25-2018, 12:36 PM

Yes, "All" power and grounds! Heat/cold cycles, add vibration and even some salt will negate good connections over time. A good voltage drop test with will help verify a circuits continuity.

Internal corrosion of a wire can take place without an insulation puncture too.

**NeedOne** · 02-25-2018, 01:27 PM

Something goofy, but for what it's worth - my 200 efi was having electrical Gremlins. the harness plug was oily.. contact cleaner and it has run like a top since

**Motv18** · 02-25-2018, 03:36 PM

The plug wires if replaced might not be shielded, check any weak breakers supplying power. Also check the main connector where the sensor leads collect gm delfi breakdown from vibration

**lost2a6** · 02-26-2018, 12:03 AM

Even though I verified a ground I suppose that it could be a weak ground. I'll guess that I will pull all grounds, clean, dielectric grease, and reinstall. Thanks for the ideas.

**lost2a6** · 02-26-2018, 12:05 AM

Originally Posted by NeedOne

Something goofy, but for what it's worth - my 200 efi was having electrical Gremlins. the harness plug was oily.. contact cleaner and it has run like a top since

Which one, the one going to the ECU or the one going to the key switch?

**NeedOne** · 02-26-2018, 07:49 AM

Originally Posted by lost2a6

Which one, the one going to the ECU or the one going to the key switch?

In my case, at the ECU.

**billybob** · 02-26-2018, 09:29 AM

The batt. cable ground also, even at the battery. if you have the old style volt reg,s without black potting. replace them.

**lost2a6** · 02-27-2018, 09:42 AM

I just unplugged the harness going to the key switch and ran it with a jumper wire and it still has erratic readings. I also removed the grounds, cleaned, dielectric greased and reinstalled, same results. I did find a problem with the low pressure fuel pump and this could have caused me missing a couple of hundred rpm so at this point I need to go to the water with it and test run. Only thing left to do that I can think of, is tying into the harness, between the plug and the ecu, with a good hot and ground to verify that I don't have a bad connection in the harness somewhere. I guess it's possible that the erratic readings are normal with this engine however I wouldn't think so.

**lost2a6** · 02-27-2018, 09:46 AM

Originally Posted by billybob

The batt. cable ground also, even at the battery. if you have the old style volt reg,s without black potting. replace them.

I'm almost positive that it does have the black potting, however I did disconnect the yellow wires going to the VR to disable them and still had the same results. Batt ground is clean and tight.

**billybob** · 02-27-2018, 10:13 AM

It may be just 1 sensor, causing problem. I,m not sure on analog motors if sensor supply voltage is 5 volt or 12 volt. Just try unplugging 1 sensor at a time & see if anything changes. that is usually a hard thing to track down. Also try unhooking tach. It,s unlikely but i,ve seen weirder things than that.

**lost2a6** · 02-27-2018, 01:05 PM

It has 12v supply to ECU, however reference volts to TPS is around 7. I unplugged the key switch harness which eliminated the tach as well with still the same results. I did unplug the TPS and temp. With engine off, all sensors look good.