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Discussion Starter · #1 ·
New to the group and hoping someone with some expertise can help. This is a long post but I wanted to provide as much information as possible.

I have a Chevrolet K5 Blazer I have set up to be my back roads vehicle. When I purchased it, there was already a Ram Jet 350 installed. This engine is slowly driving me insane. My current battle with it is I will be driving down the road and it will suddenly and momentarily cut out. It continues as if nothing happened but it is very dis-concerning. I have replaced the ECM in the truck and it did this prior to the ECM replacement. Once in a while, it would turn off completely but would restart.

A little history on the vehicle...

Engine was installed several years ago and started out as a MEFI 3 but was converted to MEFI 4 prior to my ownership. Unfortunately, it set for a very long time before I purchased it.

After purchasing, it was doing some strange things and ultimately I replaced the IAC, MAP, MAT and O2 sensors. This made it run much better.

At one point, the heads were rebuilt. After the heads were rebuilt It ran great but it started to develop this cutting out issue.

One day, going to work up in the mountains, it cut out completely and would not restart. I am unsure if this is related to the original issue. Prior times the engine had cut out but the fuel pump would still keep pumping and the Bazer would easily restart. This time I noticed the fuel pump was not engaging at all. Further inspection revealed the fuel pump had a catastrophic failure, shorting out my ECM.

The fuel pump was replaced with an Aeromotive A1000. A new ECM and complete Ram Jet wiring harness was installed. The Blazer continued having issues. At times, I would be driving down the road and the Blazer would backfire out the exhaust. Often, the Blazer would enter into some type of safe mode and reduce the engine performance severely over 3,500 RPM's. I am unsure if the limp mode caused the back fire or vice versa. There was not always a back fire but it would go into limp mode pretty often. This was after the green mode had passed.

I discovered the shop who installed the computer and wiring were unaware of the process to correctly set the timing. That issue was addressed and the Blazer runs much better but it still will cut out going down the road.

It does not run hot. I have never had overheating issues. I am at a loss and unsure what is taking place. I have spare sensors I keep just in case there is an issue on the trail. Changing out the various sensors makes no difference in the way it runs.

Any ideas? Thanks in advance.
 

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This sounds electrical in the wire harness and connectors.

Going back to the fuel pump failure, the pump should be running off a relay that the computer manages. This should be an isolated circuit for the pump with the computer doing nothing more than grounding a relay coil that operates the pump on/off switch, threre should be no direct connection between the ECM and the pump.

Bogie
 

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X2 with Bogie's comment on the fuel pump setup. The pump should be controlled via a relay on a switched ground input that controls the relay.

If you believe you have the skillset to dive into this, then you need to pinout the entire harness and verify integrity of all of the wiring. Once you have done that find the wiring instructions for the RamJet ECU - from GM or whomever it was purchased.

If you don't have the skillset to dive into this wiring, then take it back to whomever installed it and hope they have the skillset to fix it and/or determine the issue.

Personally, I'd lose the GM ECU and go with an aftermarket ECU and be done. This is an extremely simple system to install an aftermarket unit onto.

Can you get into the wiring confidently?
 

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Discussion Starter · #4 ·
While I appreciate the replies and agree, this would have been the best way for GM to set up the system, I don't think this is the problem or has anything to do with the issue at hand.

As previously mentioned, this is the second ECM, fuel pump and complete wiring harness. The engine cut out has persisted through out all steps of this process which would indicate the problem is elsewhere.

These are intended to be very simple systems with very limited wiring. There were no "factory" relays and the ECM bolts to the top of the passenger side valve cover. Unfortunately, they offer little support from GM and their diagnostics system is poor.
 

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Harness, ECM and most of the sensors have been replaced but has the same problem. Sounds to me as if there is a ground problem. When replacing the harness maybe the grounds were run to the same spot so the problem continues. Revisit the grounds throughout the whole vehicle would be where I would start.
 

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Is there a check engine light installed? Does it illuminate? On boats that run an mefi system without a check engine light, I can plug into it with a laptop and monitor engine data as well as read diagnostic codes. Before that, I would place an led across 2 terminals of the data link connector by the computer and jump 2 other terminals to put the ecm into diagnostic mode where it would blink the led light with the codes.

If you can put it in diagnostic mode, than you might get a hint of where to look. The ecm will set a code if there is any intermittent signals coming to it. If there are no codes, than the ecm is fat and happy with the sensors and their signals, and the problem is likely power or ground issues shutting down the ECU. The wiring plug going through the firewall would be a prime suspect as was suggested. You could run a new power lead directly from the ign switch to the ECU.

Is your alternator charging correctly? EFI systems need plenty of clean power to function correctly. A bad alternator diode sends ac into the engine wiring.

Sent from my moto x4 using Tapatalk
 

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Discussion Starter · #8 ·
I don't think it is an ignition switch problem or even the socket at the firewall. Everything else continues to work.

I have been chasing wiring on it since I bought it. The previous owner did some poor wiring with aftermarket accessories, most of which I have removed all together. I will track the ground(s) down and see if there are any issues there.

Good alternator and dual battery set up.

It is basically the same system as the marine set up. I have the little diagnostic tool that plugs in and blinks to give codes. All is well and things are addressed when there is an issue.

I am very curious about an intermittent short and think this may be the problem. It would have to be in the original wiring that is switched. Everything else related to the engine is independent and has been replaced at least once.
 

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Harness, ECM and most of the sensors have been replaced but has the same problem. Sounds to me as if there is a ground problem. When replacing the harness maybe the grounds were run to the same spot so the problem continues. Revisit the grounds throughout the whole vehicle would be where I would start.
Bada Bing, Bada Boom. You're reading my mind.
Motor must be grounded to body and frame.
Body must be grounded to motor and frame.
Frame must be grounded to motor and body.

You will need two of these.....
https://www.summitracing.com/parts/...1596235436771&utm_content=GSAPI+5ba2a4366829c
I have spec'd an 18" cable to insure that you have enough length to make connections. If you wish to use shorter cables, that's up to you. These are equal to a #4 wire size.

Choose a location on the motor where you can ground the block to the frame. A location at one of the motor mounts should work well. Now, choose a location on the frame where you can ground the frame to the body. In the engine bay where the frame comes close to the firewall should work well.

At connection points on the frame and body, you must sand or grind down to BARE METAL to make this fix work well.

At the motor mount on the passenger side, with a hydraulic jack under the oil pan just enough to prevent the motor from moving, remove one of the bolts that attaches the mount to the block. Use a bottoming tap (Ace Hardware) that matches the diameter and pitch of the bolt and run it in all the way to clean up the threads and remove any foreign material from the thread bore. Use a fresh, new bolt and an external tooth lock washer (Ace Hardware)to attach the cable.

The other end of that cable will attach to the frame by drilling a hole and using a self-tapping hex-head bolt and external-tooth lock washer. Use caution when drilling into the frame and be aware that tubing may have been run inside the frame rails. Drill only deep enough to clear the thickness of the rails and then use the shortest self-tapping bolt that will engage the rail thickness completely. You can grind off the point of the bolt down to the diameter of the hole if you wish.

Use the same procedure on the frame close to the firewall for your (frame to body) connection. On the other end of this connection at the firewall, we will use a through-bolt and nut, with large flat washers on each side of the firewall to prevent pull-through in the thin material of the firewall at this point. You will likely want to move up onto the transmission hump a little to make this connection, to prevent drilling into any part of the AC/heat components. Choose a location where you can get to both sides of the sheet metal to scrape/grind down to bare metal. There will be nothing wrong with attaching the ground cable at the flat part of the floor where the passenger places his/her feet, as long as it does not interfere with the placement of the passenger's shoes (make an uncomfortable bump on the floor). At the point of transition from flat floor to transmission hump would be a good place.

Slather both sides of firewall connections and one side of all other connections with clear silicone to prevent oxidation.

This write-up is not only meant for Travis, this is for anybody who drives a vehicle of any type. And particularly for off-road vehicles that would be more susceptible to degradation from moisture and jarring or the components.
 

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While I appreciate the replies and agree, this would have been the best way for GM to set up the system, I don't think this is the problem or has anything to do with the issue at hand.

As previously mentioned, this is the second ECM, fuel pump and complete wiring harness. The engine cut out has persisted through out all steps of this process which would indicate the problem is elsewhere.

These are intended to be very simple systems with very limited wiring. There were no "factory" relays and the ECM bolts to the top of the passenger side valve cover. Unfortunately, they offer little support from GM and their diagnostics system is poor.
If there are no relays in this system, then it is not wired correctly. No GM ECM of this era was designed to act as a relay.

Respectfully, you mentioned you 'don't think this is the problem or has anything to do with the issue'. I'm sure that it could be an issue or is the issue. Either way, I would recommend to confirm what is correct. Assuming that something is correct because the part was changed is a mistake when working on electrical issues.

lmsport - if memory serves me correctly, this runs on an 8 pin HEI as found in most every TBI truck and Firebird and Camaro TPI.

Adding additional grounds will never hurt anything and they take minutes to make and install. If addition of a couple of grounds resolves the issue, then you know that original ground path was the issue and that could explain the pump back feeding power to the ECM and shorting the fuel pump control circuit.
 

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GM campaigned all the small cap distributors for the reluctor magnet cracking, caused some odd driveability, including a stumble/missfire. I used to have a box with about 100 of those distributors and dealer mech gave me.
 

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Saying everything else continues to work is like ....the other 3 tires are fine only ones flat or. 7 of the cylinders work good , it only takes on deficiency to cause one problem. If you know what's good then what isn't ???
 

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GM campaigned all the small cap distributors for the reluctor magnet cracking, caused some odd driveability, including a stumble/missfire. I used to have a box with about 100 of those distributors and dealer mech gave me.
Agree. A spare dizzy ($100 from Advance Auto or a junkyard unit)
and see if the problem stops. OP has thrown a ton of parts at it with no resolve. Sure would be nice to see what isn't working when the misfire starts. The aftermarket caps and rotors are not durable in my experience.
 

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All of the marine mefi systems I have worked on have 2 relay's, one to turn on the ECU, and one for the ECU to turn on the fuel pump.

Just because you think all of the other circuits going through the firewall connector are working correctly doesn't mean that the most important one, the ign connection is good. That lead has a pretty high load on it as it not only powers up the ECU relay, but the coil, and the alternator excite lead as well.

That little module in the distributor could be failing as well. Being miniturised it developes a lot of heat. It relies on a heat sink compound to transfer that heat to the distributor body, and will fail without it (my ex partner wiped it off once). It would probably be worth changing that module out as it costs less than $40 .

Sent from my moto x4 using Tapatalk
 

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Discussion Starter · #16 ·
Ok, to try and clear up some confusion. This system does not use relays. It was built by GM that way. I agree, I wish it came with not only relays but also some type of fuse block set up. This was one of their selling points for the system, its like a 4 or 5 wire hook up.

There is a 12 volt power lead to the ECU and a switched. 1-50 amp fuse in the 12 volt main power lead. There is a wiring harness that goes to the O2 sensor, the various plugs for intake sensors and injectors and the ECM is bolted to a bracket directly above the passenger side valve cover. It comes out of a crate like this.

The only portion of wiring to the engine which is not brand new from GM is the switched power. I will track it and see where it is coming from and if there are any breaks or rubs which may cause a problem.

It has a small cap distributor and I guess there is a possibility of something failing there. I will need to figure out what distributor interchanges with it and possibly try a replacement.

I did drive it to work last night (45 minutes) and it ran great other than one little hiccup (sputter) but even that was not as bad as usual. On the drive home, it did not do it at all... It's a pretty hard run to work, mostly up hill at a pretty steep grade. I go from 3500 feet to over 7000 and drop back to 6000. It was also in the low 20's this morning and it ran awesome. Not that any of the temperatures or elevations matter but who knows with all these sensors.

I will say, resetting the timing has made a HUGE difference, more than I ever thought possible. I appreciate all the help. Monday, I will be taking this to friends shop and we are going over all the wiring on it. I will let everyone know if we find a gremlin in there somewhere.
 

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A couple of pics of what you have might help.

Regarding interchange of the distributor, pull the cap and rotor. If you have 4 pins out of the bottom of the cap, and then two separate 2 pin plugs under the rotor, then it is extremely common. Lookup any dizzy for a late 80's 5.7 Firebird or Camaro. While you're there look at the rotor contact and pins n the underside of the cap. If you aren't sure what you're looking at, then post a couple of pics of them.

Do you own a multimeter? And what is your system for setting the timing? Does your ignition module look like the one in the attached pic?

https://static.speedwaymotors.com/pdf/12499120instructions.pdf

Are you setting the timing as described on page 8 of ^^^

p.s. - I hunted for one of these FOREVER before we settled on a TPI. Congrats on having one. PICS PICS PICS
 

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Discussion Starter · #19 ·
Sorry for the delayed responses. My internet is pretty poor here. I will be tracing the ignition wires and grounds on Monday to see if I can find anything standing out. I suspect the problem is either a ground or something in the switched 12v shorting out or even a broken wire. I will keep you guys posted at the findings.
 

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That 8 pin HEI schematic that 64nailhead posted looks like the correct module. That style distributor came on various styles of EFI engines in the 90's. Not just mefi style but vortec CI and even TBI engines as well. 6 and 8 cylinder modules are the same. It's the number of teeth on the reluctor wheel that changes for 6 & 8 cylinder engines. I wouldn't change out the whole distributor, it has no moving parts except the shaft.

The ECU supplies the timing signal except during cranking where the module operates in a base static mode. After the engine reaches 350rpm the ECU sends a voltage signal to the module, and then the module starts following the ECU's timing commands on another lead. That's why the modules are the same, it's the ECU which commands the timing.

On a side note, carburated boats with that style distributor uses a different identical looking module which actually has an rpm based advance curve built in. When people replace that module with an automotive EFI module, it runs in base timing mode without any advance curve.

Speaking of setting the timing, you have to use your diagnostic tool to get the ECU to go into base timing mode to set the timing. Otherwise the ECU will be constantly changing the timing trying to find the best idle, and will usually add about 10 degrees above the base setting. Setting the timing without putting it in diagnostic mode will result in retarded timing.

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