Hi. 1999 270 DA, 7.4L MPI. Last...

[Grayorg]

Hi.  1999 270 DA, 7.4L MPI.  Last year, running open throttle, ran great, when slowed down, engine ran erratically (RPM not consistent), but well enough to use.  Before winterizing, put in fuel stabilizer in tank which was half full, then stored.  Changed rotor, cap, wires, and plugs.  When commissioning this year, would start only for about 2 seconds then die.  Put in K101 additive, waited 24 hrs, and it started and ran, although not very well.  Heard loud sucking sound from intake, thought maybe the IAC was bad so changed that.  Revved to about 2k, would not stay there, searched from 1800 to 2000.  Went back to idle and died.  Restarted and let it idle, but again, rpms ranged from 600 to 1100.  I'm thinking bad gas?  Don't have any diagnostic tools for this engine so can't check for any codes.  Any suggestions?  Thanks.

[wingless]

Welcome to the forum.

The easiest solution is to yank the garboard drain plug, then go down the street for several beers.

All basic stuff to restore normal operation. Time consuming but not bad. Probably not gas. These things will burn most anything.

Here is a partial list:

Rotor (Inspect tip and for tracks away from tip)
Distributor Cap (Inspect electric connections and for cracks)
Timing (Check / set per FSM)
Spark Plug Condition (Insulator Visual to Indicate Burn, Ceramic Cracks)
Spark Plug Gap
Measure spark plug wire resistance
Compression
Fuel Pressure (Multiple / All Test Conditions)
Send Injectors for Testing / Cleaning
Engine Fault Codes
All Vacuum Hoses Clean / Clear (Mine are Cleaned Annually)
Flame Arrestor Clean (Mine is Cleaned Annually)
New Fuel Filter(s) (Mine is Replaced Annually)
Remove / Inspect / Clean / Replace Intake Plenum (Mine is Cleaned Annually)

Here is the DDT, purchased before they became unavailable.

https://live.staticflickr.com/4357/36075822920_55df4c9044_c.jpg

[Grayorg]

Thank you! I will be checking those things today and report back with findings.

[Grayorg]

Update - everything checked out ok except the plugs were gapped at around .020, should be .045. Rookie mistake on my part to assume pre-set was correct = lesson learned! Runs great now! Thanks again.

[wingless]

Great news!

Thanks for reporting back w/ the fix.

Makes good sense. The other listed stuff was learned by me during my long term 454 MPI MAG Horizon ownership, not listed in any FSM documents, to keep the engines at 100% performance.

Good luck.

[Grayorg]

Hi - update #2 - took it out for the first time today. Ran great at 3/4 OT. When slowed for no wake zone ran very erratically (just like last year but I think worse). When put in neutral it stalled. Tried to restart a couple of times but just died right away. Waited about a minute then it started and actually ran ok even at idle. I did not check the plenum and injectors as you suggested yet, nor have I been able to read the codes (was hoping to not spring for the reader but it looks like I have to). Is there a way I can test the injectors? Could it be the ECM? Also something new I just noticed today - the temp gauge was showing 150 steadily, then just dropped to zero. After a few seconds it would go up a little bit then back to zero, and repeat but not in any consistent manner.. I’m guessing sender or gauge. Maybe related to how it’s running? Thanks.

[wingless]

Thanks for the update.

Measurement of fuel pressure is an important test.

Years ago when I accelerated my hair loss (by pulling out) I did a massive effort to attain 100% engines performance. That included sending out all 16 injectors for testing / cleaning / performance report. They discovered a bunch that were bad.

The timing on one engine was waaay off.

My fuel pressure manifold vacuum regulators were clogged, one was impossible to restore, one reverted to normal after cleaning.

Standing by my initial list for performance restoration.

The unobtainium DDT tool is handy but not required.

[wingless]

Note that Mercruiser uses those push on right angle barrel connectors for these types of sensors and my experience is that those connectors fail, providing flakey readings.

When my push on connectors have failed I snip them off and properly crimp on correctly sized ring tongue connectors, retained by correctly sized nuts and washers, onto the existing threaded sensor posts.

[Grayorg]

Update #3 - checked the sender connections, all good. Checked fuel pressure at the fuel rail - 60-75 psi at 1000 rpm. Spec says 43! Guessing fuel pressure regulator is shot, but it was idling nicely so....?

Had the marina mechanic connect their diagnostic tool - no codes except one overheat, but I know that was from last year (ran aground in the Great South Bay of Long Island - not uncommon unfortunately!).
The diagnostic tool read the temp at 165, the gauge didn't read at all, I'm guessing the gauge is shot, but I'll check the connections before replacing.
MAP voltage at 1000 rpm was 1.95. Again, this was all done while it was NOT previously run for 20 minutes or so at 3/4 throttle. The issue only seems to occur after running at med/high speed for about 20 minutes or so. The TPS also checked out ok. Could this all be related to the pressure regulator? Thanks again.

[wingless]

Thanks for the update.

My understanding is the fuel pressure regulator only knocks down the normal full throttle fuel pressure (~43 psi) to lower when idle manifold vacuum is applied to the port. It won't decrease abnormally high to normal.

My suspicion is that the pressure is okay, and instead that the gauge is just reading high.

Your gauge or the mechanics?

When I fixed / checked mine I followed the FSM and also used my hand vacuum pump to apply vacuum to the regulator, plus no vacuum, atmospheric pressure, to verify the regulator is working fine.

Don't be fooled by the engine sensors / switches. There should be two each for water temperature and for oil pressure, four total. When I was fixing mine, the problematic one was not initially easy to locate.

[Grayorg]

Update #4 - turns out the fuel pressure regulator was bad - changed it and runs great now!
Looks like the issue with the temp is the thermostat - changed the gauge and sender and no change. I replaced it in the spring but I guess it's getting stuck open. I'll pull it and see if I can check it in some hot water. Thanks again for all your help Wingless.

[wingless]

Thanks for the update.

For the bad fuel pressure regulator, both of mine went bad because of the poor engine design, using manifold vacuum, drawn from a port on the bottom of the intake plenum.

The small amount of engine oil mist (blowby) accumulates in the plenum, then drawn down the tiny regulator hose by capillary action, to plug the regulator port.

I was able to restore normal operation to one, but the other was stuck on low pressure.

It is "easy" to check the regulator per the FSM using a hand vacuum pump applied to the regulator port. Vacuum applied (simulating idle rpm) should decrease the measured fuel rail pressure.

Now I clean the interior of the engine plenums and clean those vacuum hoses on an annual basis.

Zero problems since that became a normal maintenance item.