7.4L MPI Gen VI V8 Power Drop Off

[BryanOnTheBay]

I have been having a weird problem with my engine acting like it is starved for fuel at certain RPMs and it is time to see if there is something not obvious that is causing it. The engine is a 7.4L MPI Gen VI GM 454 V8 engine. The current symptoms are:
1. At 2600 RPM, the engine starts popping and losing power to accelerate. It feels like it is being starved of gas. It can be gently accelerated through until about 2850 RPM where is will gain power again about half the time, the other half it will falter and drop back. Further increase in the throttle does nothing if this happens.

2. Somewhere past 3250 RPM the same thing will happen prior to 3600 RPM. If this happens, the engine will generally back off to 2600 RPM. where this occurs varies in that range.

3. In order to gain control of the engine again, I have to back off on the throttle until the engine RPM jumps up, and then I can accelerate again, subject to the above constraints.

What has been done ( all parts OEM unless otherwise noted)
1. New plugs;
2. New plug wires;
3. New distributor system including, cap, rotor and body;
4. New ignition coil (AC Delco, not mercruiser);
5. New fuel pump (it failed during all of this);
6. Fuel injector cleaning;
7. New fuel filters;
8. Compression test;
9. Fuel pressure test.

Anyone have any thoughts? My concern is that it is some weird gremlin.

Bryan

 

[scoflaw]

Did you check that your timing was advancing properly at speed ?
What was your fuel pressure reading when failure was occurring?

 

[BryanOnTheBay]

Did you check that your timing was advancing properly at speed ?
What was your fuel pressure reading when failure was occurring?

I haven't checked the advancing of the timing. How is that checked? It was timed by the mechanic.

With regard to the fuel pressure, I am not sure as I was driving, but the mechanic said it was normal pressure

Bryan

 

[scoflaw]

Digital timing light with a dial on the back of it, that reads rpm's as well.
Fuel pressure was good when the issue occurred?
When you say dist. body are you saying whole drop in unit? Check your timing advance

might want to disconnect the tach wire from coil that could be the cause as well

 

[BryanOnTheBay]

Digital timing light with a dial on the back of it, that reads rpm's as well.
Fuel pressure was good when the issue occurred?
When you say dist. body are you saying whole drop in unit? Check your timing advance

The mechanic has to love that...

The fuel pressure was good throughout. I would have had the injectors cleaned at that point if it hadn't been done within a couple of service hours prior to that test.

And yes, the entire drop in unit for the distributor.

Bryan

 

[scoflaw]

That light is cheap enough at Auto Zone, he should have one, how else does he check total timing.
What's up with the non stock coil? Is it good enough?
Try taking that tach wire off for a test run, it happens.

 

[fc3]

What's popping? Backfire or a simple miss? Once the engine starts, I think about 350 RPM, the MEFI module takes over the ignition from the HEI unit in the distributor body. It is not a bad idea to check the ignition timing at the suspect RPM levels.

 

[BryanOnTheBay]

What's popping? Backfire or a simple miss? Once the engine starts, I think about 350 RPM, the MEFI module takes over the ignition from the HEI unit in the distributor body. It is not a bad idea to check the ignition timing at the suspect RPM levels.

It isn't a backfire. It sounds like popping from fuel starvation.

Bryan

 

[BryanOnTheBay]

Beuller? Beuller? Has anyone seen Ferris Beuller?

Seriously, does anyone else have ideas? I want to go through as many as possible as quickly as possible with the mechanic. Thanks!

 

[fc3]

I'd do two things. I'd monitor fuel rail pressure during the time the engine is misbehaving and also monitor the MEFI inputs at the same time. You might have a wonky sensor particularly TPI and MAP, or there might a fault telltale set. I'm pretty sure that MEFI on the 454 does not have fuel fuel rail pressure sensors but I think the small block V8 does. I forget

 

[fc3]

Here we go!

http://www.marinemechanic.com/merc/distributors/mercurymarine/sterndrive/mefi3-troublecodes.pdf

No fuel pressure senders. You need to use a direct reading gauge attached to the schrader valve port.

 

[Jimmy Buoy]

How were the compression test readings? A buddy of mine had similar symptoms on his 7.4 and it was a cracked head which allowed cooling water to enter the cylinder but only when he pushed for more power. I would think the compression test would reveal this though.

 

[BryanOnTheBay]

I'd do two things. I'd monitor fuel rail pressure during the time the engine is misbehaving and also monitor the MEFI inputs at the same time. You might have a wonky sensor particularly TPI and MAP, or there might a fault telltale set. I'm pretty sure that MEFI on the 454 does not have fuel fuel rail pressure sensors but I think the small block V8 does. I forget

We had a pressure valve on the rail, but did not have the MEFI monitored. That being said, there have been no codes at all. I don't think the 454 has fuel rail pressure sensors either, at least not in the Gen VI.

Bryan

 

[BryanOnTheBay]

Here we go!

http://www.marinemechanic.com/merc/distributors/mercurymarine/sterndrive/mefi3-troublecodes.pdf

No fuel pressure senders. You need to use a direct reading gauge attached to the schrader valve port.

We did. It was a good reading. Thanks for the service bulletin!

Bryan

 

[BryanOnTheBay]

How were the compression test readings? A buddy of mine had similar symptoms on his 7.4 and it was a cracked head which allowed cooling water to enter the cylinder but only when he pushed for more power. I would think the compression test would reveal this though.

Compression test was good in all 8 cylinders. Actually impressive for the age of the engine.

Bryan

 

[sdarc]

I had a similar issue when I had my 270. It turned out there was a fuel shut off inline right above the tank. Not a gate valve but like a very small house spigot. Seemed the vibration caused it to start closing. Worth a shot.

 

[LTD.330]

This is a reach but may be worth looking into. How clean is your flame arrestor? A dirty flame arrestor would starve the engine for air and the symptoms would be similar to fuel starvation. The computer may lean out the fuel to compensate for reduced air flow and you get engine knocking.

 

[BryanOnTheBay]

This is a reach but may be worth looking into. How clean is your flame arrestor? A dirty flame arrestor would starve the engine for air and the symptoms would be similar to fuel starvation. The computer may lean out the fuel to compensate for reduced air flow and you get engine knocking.

The flame arrester was cleaned last year and checked this year.

Bryan

 

[BryanOnTheBay]

I had a similar issue when I had my 270. It turned out there was a fuel shut off inline right above the tank. Not a gate valve but like a very small house spigot. Seemed the vibration caused it to start closing. Worth a shot.

The new mechanic was out this evening. You are closest to his working diagnosis. It seems the fuel line coming off the back of the pump had developed a kink, and it was restricting the fuel flow. When my fuel pump was replaced last year, the fitting was not well aimed and made the condition worse. This would help explain the slow worsening of the problem as well as the lean symptom.

The mechanic shifted the fitting so it aims better, and the RPMs jumped up, but there is still some restriction until the hose is replaced. The next chore is finding a new hose. If not, we will have it made custom.

The reason it was missed is that the braided jacket was not kinked, so the mechanic had to feel the kink through the braid.

Bryan

 

[BryanOnTheBay]

The new mechanic was out this evening. You are closest to his working diagnosis. It seems the fuel line coming off the back of the pump had developed a kink, and it was restricting the fuel flow. When my fuel pump was replaced last year, the fitting was not well aimed and made the condition worse. This would help explain the slow worsening of the problem as well as the lean symptom.

The mechanic shifted the fitting so it aims better, and the RPMs jumped up, but there is still some restriction until the hose is replaced. The next chore is finding a new hose. If not, we will have it made custom.

The reason it was missed is that the braided jacket was not kinked, so the mechanic had to feel the kink through the braid.

Bryan

i just want to close this thread out. The problems were indeed from the kinked fuel line. We are good to go now. Thanks, all, for the assistance. I hope this thread helps someone else.

Bryan