New engine will not throttle up past 1500 RPMs
 

So after losing my first engine with 700 hrs because of 2 bad injectors I install a rebuilt 350 mag MPI long block. With all original peripherals from the old engine.

Original Engine Serial Number M350MPIB3 W042371


On first start up the engine idols perfectly. But when trying to throttle up anything over 1500 to 2000 rpms the runs rough surging drastically.

So first had mobile marine mechanic out with mercruiser computer he had no answer to the problem.

So another mechanic came out and suggested to disconnect the TPS and wow the engine throttled up with no hesitation. I replaced and check voltages at the new TPS and checked out fine. Yet the engine still will not throttle up.


Things that have been completed


1 New cap and rotor

2 New oil pressure switch

3 Plug wires tested and verified firing order

4 New Plugs

5 Monitored fuel pressure 40PSI

6) All injectors tested

7 Checked compression all 90 psi low but engine has only run for maybe 1 hr so I hope break-in will improve compression.



Questions

1 Thinking it is electrical problem

2 The TPS must be seen by the ECM or it would not run better when disconnected

3 Could it be a bad map sensor? I tried disconnection map sensor and it started but immediately stalled

4 Could it be a bad ECM?

5 Does the TPS need some sort of calibration for ECM to see it properly like in some cars?

6 Could a rebuild be far enough off where the original ECM can't run it properly?


Thank for any Ideas

Joe

if its a flat cap dist try turning it one way or another i use a cut open cap while running to get it lined up

This is concerning !!!!! Get another psi tester to confirm !!!

Yes I am also concerned original engine was 180psi with same gauge. But I think the low compression would affect the the max power output but not the rough running condition. Since when TPS is disconnected it throttles up smoothly.

New TPS?
Tip: if you can find it Amazon or Walmart (online, many vendors)...
...you can return it if needed.

Ayuh,...... Welcome Aboard,...... Lack of fuel killed the 1st motor, so I'd start there,.....

Start at the fuel tank, through the dip-tube, anti-siphon valve, the fuel lines, fuel filter, all the way to the injectors,.....

Yes already replaced TPS and confirmed it working correctly. Checked output voltage .6 at closed throttle and 4.75 at wide-open throttle

Modern engine 'break-in' is not much of a thing anymore.

The exception being FT cams.

Ring seating is done very quickly if everything setup correctly...bonus if you gey a bore plateau hone.

Modern rings are very smooth...modern bores are very smooth...they use cross hatch in the bores to lube.

Ring/bore break-in is now more of a cleaning action. Micro-burrs coming off and getting cleaned away etc.A lot of this can be avoided during bore cleaning and setup. Green Scrubby bore finish etc

As SB mentioned...90 psi is not just low...you don't have an engine. Do you increase PSI if you put oil in the chambers with a squirt lube gun? Maybe you got fuel washdown and messed up the rings?

Cam timing off= low compression

Any way to check without tearing front end down?

How did you time the engine after installing the distributor and electronics

After setting number 1 cylinder on TDC install distributor with in the number 1 firing position. Making sure when fully inserted it is still in alignment. This is the first thing my mobile mechanic checked.

Are you positive the old engine and new engine have the same exact camshaft? If they don't, the ECU will need to be recalibrated.

That would not cause the issue he is having.

If you didn't put the ecu in base timing mode then set the timing then remove from base timing mode its not done correctly...........

If it has the flat cap , that is not the procedure. Lots of 350mpi’s have the flat (crab) cap. If it has the est/voyager dist, then your procedure is typ correct.

This pic is the flat/‘crab’ style
https://cimg6.ibsrv.net/gimg/www.off...390979e82.jpeg

Btw, and again: 90psi is horrible. :(

Agree, ya I got BBC on the brain didnt mess with the mouse motors

do you still have your old spark plug wires?if so,put them back on and see if it revs up.I had a 350 that did something similar,turned out the spark plug wires were not seating in the cap,put old wires back in and up on plane we went.just a thought.

Joe,

When you try and throttle up, what does your exhaust output look like...?

Understand first that the TPS is essentially nothing more than a movement indicator. It does not tell the ECM what speed you want to be running, but rather, when you want to change speeds. The ECM commands fuel based on MAP and RPM. When you shove the throttle forward, the throttle blade opens, providing more air while the RPM and MAP remain essentially the same (for conversation's sake), so the AFR must be corrected momentarily with a shot of extra fuel. So, the TPS commands "acceleration enrichment". I'm going to assume the same process, in reverse, occurs when you back off the throttle (I am a total layman, who has learned a few tidbits through osmosis, being in proximity of an EFI genius working on my engine's tune).

So, after rebuilding my 496 last winter, where I replaced the PCM555 with a Holley HP, we noticed a drastic increase in fuel consumption. As in, WAY more than any predicted gains in HP we expected from the various improvements we implemented during the rebuild to have incurred. Having full access to the tune via Holley software, the tuner was able to discover a LOT of electrical noise coming from the TPS, which was triggering acceleration enrichment, pretty much constantly. This was the primary culprit for the excessive fuel consumption, as well as some minor behavior gremlins we couldn't seem to tune out. After installing a new TPS, the problem initially seemed to be resolved, but then it returned. Not as bad as before, but still remained. The tuner pulled the connecter and discovered the retainer/locator sleeve on the connecter had been lost, so the connecter, while firmly clicked into place, was not securely tight; it could be wiggled. After correcting this issue (spliced in a new connector over the winter, while we had the engine out again to address a coolant leak and swap a cam, etc, etc...), and after first fire this year, the electrical noise is gone.

Long way around to get back to your engine's behavior gremlin going away when you disconnect the TPS.... In many cases a bad signal is far worse than no signal at all. Without the TPS, the ECM is just going off the fuel table, and it's just winging it through throttle changes. But a TPS throwing a bunch of noise will cause your accelerator pumps to have a field day. Ensure all connections are positive and secure and see if you can find a way of detecting said electrical noise. On the Holley, it was easy. The tuner could just look at a graph of the TPS signal output. It should have been a nearly flat line that smoothly followed my throttle sweeps. Instead, it looked like a crackhead's EKG. This sounds a LOT like where you are.

Something to chew on.

Thanks. Brad.

Still having problems
I installed a new tps and map sensor no change. Also checked output from TPS and it is stable and the correct voltage like .6 to 4.5 volts. I tring to get a moble mechanic with the mercruiser G3 program out to see what he can see with the computor connected. Any additional ideas are welcome. Does anyone have the pin layout for the ecm so I can check voltage at the ecm plug?

Thanks
Joe

If new issue after putting in new engine, could be bent or loose pin in ecu or any other harness connection.

So……with all this engine running did the compression go from 90psi back to healthy normal 170psi or so ?

Have not rechecked the compression. Do you think it would hard start with that low of compression? Also with tps dissconnected throttle up seem vary solid and quick response.

joefran
you said you replaced the plugs, for the hell of it put in a new set.......
i never had a "set of plugs" issue beyond an occasional bad plug with any of my boats but back in the 70's with my Z-28 i got a bad batch(s) of plugs....
i worked as an aircraft A/P mechanic at the time and when I tested the plugs on an an aircraft plug tester that applied compression and spark to the plug, only 2 or 3 fired over 50psi applied........ I got a new set from the same parts store and had the same issue, turned out to be a bad run of Champion plugs at the time.
if not the plugs, see if you can borrow a carbed intake manifold and run he engine old school
good luck

Joe,

I don't know what your exact problem is, but it originates from this right here. Do you have a way of checking AFR? Source a set of WBO2 plates and sensors and find out what's happening with and without your TPS connected.

Thanks. Brad.

if I am reading this right the original engine was a 305 and now it is a 350 but is using all the parts from the 305 except block and heads. this also appears to be a flat cap distributor motor. if so is the dist in correctly. you set it to TDC of #1 and then install the dist with the rotor pointed to the mark on the body and lock it down as the ecm handles everything else. trying to make sure what parts are used as there was a time where some components switched the wiring. I know on certain MAP sensors they swapped the + and - wire location at a certain point depending on what MAP sensor you have but not sure if that happened with the TPS as well.

you talking about this distributor set up,

https://cimg4.ibsrv.net/gimg/www.off...452d491347.jpg