Can hot exhaust manifold cause intake backfire?

[Deepwtr]

I have a new to me 1997 VIP. The interior was completely redone and supposedly the engine completely zero hour rebuilt before I got it. Everything on/in it looks really good.

At any rate I took the boys out this afternoon and it developed an intake manifold backfire when trying to throttle up from low rpm. It would work it self out usually and run fine at the top end. So, in stopping to look things over I noticed the port riser was hot hot. The starboard riser was fine. I felt them the first time we went out in it and they were ok at that time.

In searching the forum the most likely culprits are low fuel pressure and mechanical issues such as valves (scary thought). However I didn’t know if the manifold got hot if that could cause a problem?

i guess it would be helpful if I described my engine. 1997 Mercruiser 5.7 EFI. Outside of this suddenly new issue it seems to run really good.

Thanks everyone!

 

[Bondo]

Naw,.... Probably a lean condition,.....

Check the contents of the fuel filter for anything but clean fresh gasoline,......

 

[Deepwtr]

Ok, that’s what I thought. Side question on the manifolds, how hot is too hot, and what is acceptable? Basically is there a temperature where you say that is above xyz threshold, something is wrong? More scientific than the hand test?

 

[Bondo]

Ayuh,..... Runnin' hard, 160*, maybe 180* when really hammerin' it,.....

'n No, they're almost never the same temp,.....

 

[thumpar]

Since it was just built I would check valve lash.

 

[Deepwtr]

Are these Merc’s not outfitted with hydraulic lifters?

 

[thumpar]

Are these Merc’s not outfitted with hydraulic lifters?

Yes but I had the valves too tight on a TBI motor and it backfired when trying to accelerate.

 

[Deepwtr]

Ok, don’t hang me out to dry as I’m not the most intelligent with engine internals, but I didn’t think there was lash adjustments with hydraulic lifters? That’s sort of what I was getting at.

After I bought this boat, I’ve had a range of opinions on the engine. The shop I took it to service the outdrive said these engines were junk. You read around and see a lot of comments that these GM small blocks are somewhat bullet proof. I think the ok vs better version comes down to vortec heads or not maybe?

To me it seems to run good and scoot the boat right along, and really not use that much fuel.

I picked up up a new fuel filter/water separator today while I was in town. I will change that out this evening. Honestly shame on me for not changing that out myself first thing when I got the boat.

Bondo, thanks for the manifold temperature guidelines. Mine might actually be ok. I just felt them and one was fairly cold and one was “hot”. How hot I’m not for sure, but I will run it again this time armed with an IR temp gun to be scientific. Anyone ever put thermocouples in/on the risers to actually monitor them in real time just like any other gauge?

 

[thumpar]

I have the same engine in my boat. They are great. There is a proper rocker tightening to them but it has been a long time since I did it. I just remember the backfiring the first time I tried it. Fixing the rockers fix it.

 

[Deepwtr]

Ah, now I feel a little dumb. Preload for the lifters:

 

[Deepwtr]

A couple of follow on questions. I was reading about the ECU on these and it was said can engine scanner could read codes etc. Would any ol automotive scanner work or does it have to be Merc specific?

As for the valves, if they were the problem would the intake backfire be all the time or could it still be intermittent?

 

[thumpar]

You can read the codes with a paperclip and 12v led. Auto scanners don't work.

Copied from a search but I have used it.

DIY Code Reader

Materials needed:

One 12volt resistor type LED light - Standard LED nothing special make sure it is a 12 VOLT

One small paper clip cut in half

That’s it!

This home made code tool works every bit as well as the one I paid $50.00 for. It will work on most 1993 to 2000 marine EFI systems with the ten pin Data Link Connector. This includes MerCruiser, Volvo, Crusader, PCM, Indmar, and a few others.

How to hook it up:

1) Ignition key “OFF”

2) Remove the cap from the Data Link Connector (DLC)

3) Slide the LED into the female terminals of position E and F on the DLC, making sure that the positive side goes into terminal F and the negative side into terminal E. No damage will occur if you get it backwards, it just won't work.

4) Turn the ignition key to the “ON” position. The LED should come on steady.

5) Insert the half paper clip into terminals A and B – This puts the engine in “Service Mode” and codes will begin to flash on the LED. On 1996 and earlier engines you may hear the fuel pump come on. If not you should be able to hear the Idle Air Control (IAC) motor move. Use care in this step, do not insert in the wrong terminals or ECM damage could result! Look closely, the terminals are clearly marked.

6) Read codes by observing LED flashs. If the self diagnostic system is working it will flash code 12 – one flash, pause, two flashes, long pause – it will repeat three times. If other codes are present they will flash in order of lowest to highest. Continue to read codes until the code 12 sequence is repeated.

You can also be clear codes (by moving the throttle to 100% and back) and set base timing while in Service Mode.

 

[Deepwtr]

Ok, small update. I pulled the elbows off and they look great! Next to no corrosion in them, no rust flakes.

However, the gaskets between the manifolds and the elbows are not the full flow gaskets. So, I was wondering which gaskets are supposed to be in there? When I looked up the gaskets on the Mercruiser parts page per my serial number I’m pretty sure that was the full flow gaskets.

Which one is it supposed to be?

Per this article I should use the full flow gasket.

https://www.perfprotech.com/blog/articles/marine-exhaust-tips

However when I dig deeper into the cold vs warm manifold there is a picture of the restrictor gasket. Never simple.....

 

[Deepwtr]

Update on this two fold saga. Come to find out, electronics might be involved in missing, stuttering and backfiring. I put my new tach on and got some new information. Every time it wanted to stutter or backfire the tach would go crazy. For example I might be running 2500 rpm and not touch anything and suddenly it would start stuttering and the tach would jump up to 4000 - 5000 rpm. My hypothesis is a bad cam sensor. Anyone in agreement on that? I can’t seem to find it on Mercruiser parts.

As for the exhaust manifolds, the manifolds and risers looked great when I took them apart. However, the starboard manifold still got pretty dang hot. (I forgot my IR gun). I couldn’t hold my hand on it at all. Seemed to hot. Could it be possible the thermostat housing is plugged up? The engine goes up to 175 and sits there every time.

Cant figure that one out. It is a brand new impeller in the alpha drive.

 

[thumpar]

These don't have cam sensors.

 

[Deepwtr]

Oh, ok. Crank sensor then? How does the ECU pick up the engine speed?

 

[thumpar]

Oh, ok. Crank sensor then?

Nope. No crank sensor either. It is done in the distributor.

 

[Deepwtr]

I think I found it at about the same time as you posted. Just assumed it was a sensor like most of the rest of the world. This is a pretty good document.

http://marinepowerusa.com/2014/wp-content/themes/mpusa/library/pdfs/MEFI-1-2-ServiceManual.pdf

So, I have a couple thoughts/questions. I found a black ground wire just hanging out on the back of the engine coming from the wire harness. Should that by chance go to the ignition module? They show it in the manual, but I can’t really see where or how it could be grounded there unless it needs to come out of the distributor and be connected?

Do the symptoms I have described seem like something funny with the module? It is just weird how it is intermittent and the tach goes all out of whack when it runs bad.