5V reference. What's ok?

[SpareParts]

I'm working on a 94 TBI engine 5.7 gm.
It will start then die and i can't figure it out. The only thing i can find possibly wrong is the 5V reference to the sensors.
Supposed to be 5V but i measure 5.22V
Things very a little so 5V can't be a hard number but i have spent a couple of hours looking and i can not find what is ok
Does anyone know the spec for + - for the 5V reference voltage?

Problem seems to be centered around the map sensor.
With the map hooked up it starts and runs for about 10 seconds and dies.
With the map unhooked from the ECU it runs although a little rough as you would expect.
If i use a vacuum pump and supply the map with about 10/15 pounds of vacuum it idles perfect.

Tried a new map sensor and a known good map sensor. Exact same symptoms.
Tried a different known good ECU exact same problem but i did not check the 5V reference like i should have.

 

[Fless]

I hope some of this helps, and I know that we have some with good experience that will chime in.

I'm certainly not well-versed in the NBS models, but the output of the MAP sensor depends on vacuum and altitude. Looking at the output chart, it seems that 5.2v would be the highest it would put out, since I don't think it has any way to boost what the ECM provides.

Manifold Pressure/Vacuum Sensor: Specifications — 1994 Chevrolet Blazer Full Size V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1994 Chevrolet Blazer Full Size V8-350 5.7L.

charm.li

Not sure what you're working on exactly, but here's a test procedure (note the comment about color-keyed inserts):

MAP Output Check - ECM Charts — 1994 Chevrolet Blazer Full Size V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1994 Chevrolet Blazer Full Size V8-350 5.7L.

charm.li

MAP Output Check - PCM Charts — 1994 Chevrolet Blazer Full Size V8-350 5.7L Service Manual | Operation CHARM

Detailed repair manual for the 1994 Chevrolet Blazer Full Size V8-350 5.7L.

charm.li

 

[SpareParts]

So here is the thing. It's my Doctors boat. 1994 Mastercraft. Essentially it's a 1994 Chevy 5.7 TBI engine that runs in speed density as there is not a O2 sensor.
It is 32 years old and never had anything wrong that needed a mechanical repair.
He has had it at 2 boat shops and a mobile boat mechanic. 1 boat shop told him to replace the engine with something not TBI.
Every single part has been replaced by either a boat shop or the mobile boat mechanic. TPS, MAP, IAC, fuel pump, fuel filter, fuel pressure regulator, ignition module, coil, injectors,
Knock sensors are worn but that should not have any effect idling with no load.
Cap rotor, wires, plugs all good.
He has another boat that is nearly a twin to the non running one.
I swapped the ecu, same exact problem. Swapped the map, same exact problem.
I did notice vacuum is a little low but steady and drops as the engine slows and dies.

Just thinking through this i wonder if it has bad intake manifold gaskets causing the low vacuum.
Think my next step will be timing, compression check and verify vacuum is low.

 

[Eman85]

Has anyone pulled the throttle body and replaced the gasket and cleaned the ports?

 

[exp500]

Fuel pressure check also. Repaired regulator one clue.There are many different regulators by engine. Check pressure and return line.
Next check IAC per S/M. Disconnect is quick check.
Manuals here- Free Car Service Manuals from LEMON Manuals

Reference signal also depends on battery voltage as it is a voltage dropping circuit.

 

[SpareParts]

I have checked nearly everything electrical i can think of. Battery is fully charged.
Can't check fuel pressure as im not buying the $100 adaptor but it runs ok when i manually operate the MAP sensor with a vacuum pump.
All the sensors are fine. I have checked all the readings they are within spec.
He said when it was hooked up to a Mercruiser scanner there were no codes.
I have assumed having it looked at by 3 different people they would have checked the engine over.
I did have a vacuum gauge on it and when it runs the vacuum is low but i did not really pay attention i was more making sure the MAP with getting a vacuum signal.
Throttle body was off and a new gasket was used when putting it back on i was told

I now do not think it is an electrical/sensor problem at all.
I think the engine is not developing enough vacuum to idle. Now i need to drag a whole different set of tools over to his place and check the engine out itself.

 

[exp500]

Napa can fix you up on fuel fittings Make your adapters.Think fuel filter.
IAC is misunderstood by many mechs as it is not an instant change. It is a motor driven cone shaped screw, so there is a bit of searching while calibrating. RPM too low eng dies. Still not calibrated. MUST run cal procedure fully. (only a couple minutes).
Those two are what I would do first. Can clean IAC with spray and straw effiectivly if needed.
Low Vac- provide numbers and RPM. Same with Idle. Check timing it could be retarded like the parts changers that attacked the boat.
You didn't mention the Brand of replacement parts. It matters.
Post your findings, we will walk you thru to help your understanding.
Read up on IAC calibrarion.

You also never mentioned the Basics- last time running right, last tune up,

 

[West 1]

I can share that the ECM on the old boat engine is unique. It is set to run with very narrow parameters. If you change cams it will never run right again because the new cam profile alters the vacuum curve the engine is looking for.

It does sound like you have a vacuum loss somewhere. You might do a smoke test and see where the leak is? On the GM 5.7 it is possible for the intake gasket to fail under the manifold and suck some oil into the intake ports. Gives a vacuum leak and oil consumption.

The 5.7L is well known for intake gasket failures.

 

[exp500]

Just a thought on other possibilities- Have you checked vacuumn line routing? Especially to MAP.
Do you have a fuel pump control like those firewall mounted types usually on these trucks?

 

[waveryd]

I would definitely look at the IAC and intake gaskets. Tree pollen is responsible for killing many IAC'S where I live. I would also check/verify vacuum with a gauge.

 

[SpareParts]

Finally got back to the boat this morning.
At idle it is only making 11 pounds of vacuum at 800 rpm.
I checked timing. Spot on.
Compression is a little low on all cylinders and read between 130/135 psi. Mercruiser calls a minimum of 100 psi or 125 psi depending on the manual it seems.
I'm sure it's an intake leak. If i unhook the PCV valve and plug the hole's with the oil cap off there is no blow by and i sware i can feel a small vacuum before it dies.

 

[exp500]

Was the timing disconnect used when checking timing? RPM hi at idle.Was it in gear?
Suggestion- if running good (marine) fuel, advance timing a few degrees to observe vacuumn. If you are in ballpark should be 16 " or so. If it ever had a replaced dist. timing suspect.
Which manuals are your preferred for this boat? Asking because several "normal" checks for trucks seem to not be addressed.
Timing and vac only checked at idle? Compression not far out of expected range. I would think 135-145 expected with TBI heads. With variance so limited (5# is perfect) I would have done wet test too, since you suspect something not yet found.
How many hours on engine?
Last question today- Did it run steady at 800RPM or die after 10 sec?

 

[SpareParts]

Yes, to check timing engine was in service mode and checked at 1800 rpm per Mercruiser manual. I did try to advance the timing a little. No difference.
Engine has 1200 hours. I would expect the compression to be above 145 psi and more like 150 to 170 psi but 135 is still decent enough.
Revved to about 2K it is pulling ok vacuum (can't remember exactly) but idle it is not. I did check for bad intake gaskets and with the engine sealed it does build pressure.
I only have about a hour and a half or so to look at it when i get over there so i can't test everything just the next thing on the list.
It starts and gets to 800 for about a second then dies after maybe 5 seconds. Vacuum never rises above 11 ish psi.
I can make it run with the MAP unhooked (runs rough) or manually with a vacuum pump hooked to the MAP at about 18 (ok ish)

I'm kinda thinking flat cam. With the engine cranking during a compression check it gets enough air to check good but when running it just can't suck in enough to run.
Need to take the valve covers off to check that i think.

 

[exp500]

Have you reset the IAC? Flow test fuel pump?
Idle speed at 800, is that per manual? I would expect 650ish. The heads used on these engines rarely built to 9to1 comp.mostly 7-7.5 to use a representative number, so I expect 145# max.
Advancing timing should raise vac instantly. Along with Idle speed. Any Ideas why that didnt happen?
What happens when you unplug IAC?

Flat cam you can hear while running.
Any ported vacuumn lines? Other than full vac or on a temp switch?
Any EGR on this engine? Check stuck open.

 

[SpareParts]

No EGR.
No ported vacuum. Just 1 line from the TBI to the MAP and its full vacuum.
Generally you can hear something but boats are a little different that autos. If they are running half decent you can't really tell they are not until you get them on the water and under a load.
Fuel pump is fine and was replaced with new. Same exact problem.
IAC was reset and i can watch it operating. Everything on and controlling the engine has been replaced with new. I tested the ECU by swapping with an identical boat that does run.
All electrical is ruled out i believe.
A boat shop had it on a scanner and it showed nothing wrong. They told him to put a different engine in it with more updated electronics. So im told.
I don't think it jumped a tooth on the timing gears as the distributor is not turned to far one way or the other and i can time it close to what looks like a factory mark at the dizzy.
If it's not a flat cam im just out of ideas.

 

[exp500]

When in open loop, ECM relies on RPM, ECT, MAP. If coolant improper it would likely hunt RPM leading to IAC not keeping up. IAC reset is key off 10 sec/Run eng 5 sec/ignition off 10 sec./repeat as necessary.(the 87 and up trucks were picky at hi miles and had a plug unplug IAC reset that worked well if completed.)
Another check is if Fuel pump relay is weak or occasionally shutting fuel off so low pressure after running a bit.
Another item rarely mentioned is injector wires on both injectors must be twisted, Don't remember exactly but 6 per foot comes to mind.
DTC33-MAP v hi
DTC34- MAP v low
As far as dist position, look inside when on #1. 15 degrees is alot when 45 is 1/8 of 360. probably too much for little computer to compensate for. One manual lists 0 another 10 deg adv. I don't have your serial num. Again, the lack of response when you moved it before is a big concern for me.
A failing coil or a bad trigger signal would throw all the sensors and fuel you need off.
Look at troubleshooting unstable idle in truck manual, it has much more info including DTC references.
Are you able to short DTC connecteor to ground to read codes from check eng light?
Good Luck.

 

[SpareParts]

It has low vacuum there is no doubt about. No matter how much carb cleaner or water i sprayed around the TBI or intake i could not detect a vacuum leak.
Figured i would get the intake off and check the gaskets under the intake even though the engine was building pressure when sealed.
Took the TBI off and the Adaptor to find this.

Keep in mind 2 marinas and a mobile boat mechanic did not find this and told him he needed a new engine with updated electronics.
They had replaced the IAC, Fuel pump, TPS, IAC, Fuel filter, Flushed the fuel tank, New injectors and probably something else i cant remember.
So far at least a couple thousand dollars and no fix.

I ain't the smartest guy so according to the symptoms he described to me when at the lake and not using a fake a lake. Boats never run the same on a hose!
It started as dying when up to temp but ran ok cold. Slowly got worse till it would not even run for more than about 10 seconds.
Sounds like the ignition module.
Changed that and made no difference. Thinking it had to be something simple still i put a new FP regulator in it. No change.
Messing around with a few things i noticed it had low vacuum.
That's the problem and always has been.
Sprayed about an entire can of carb cleaner abound the intake and TBI gasket but nothing changed. Probably because it would not run long enough or it had too many leaks.
Took the TBI off and gasket was good. Took the adaptor plate off and found the bad gasket above.
$22 bucks in gaskets and im sure it will run great!

 

[SpareParts]

When working on boats you never know what Mercruiser does until you actually check things. Engine is rated at 275HP.
The heads are #14096217, 64CC chambers.
The intake manifold is from around 1973 as it has the place for a stove choke. Uses an adaptor plate to bolt on a TBI setup.

His other 1994 Has the Corvette LT1 350 engine in it and is rated for 319HP

 

[exp500]

Tenacity! Great work!

 

[Eman85]

Has anyone pulled the throttle body and replaced the gasket and cleaned the ports?

Never trust what anyone tells you! IAC counts should have been way off usually first clue.