Help! Can't get my new EFI 351W to idle correctly

**Duke Nukem** · 06/16/07, 04:18 PM

OK, I've got my 351W installed and I just can't seem to get it to run right.

The Project:
- Replaced my existing fuel injected 302 with a fuel injected 351W

The Symptoms:
- Starts OK and runs so long as I give it some throttle, but as soon as I try to back it down under 1,000 rpm it sputters and dies.
- If I try and rev then engine higher it pops and doesn't run well. There is also a lag in throttle response. If I stab the gas it hesitates and then starts popping and running poorly. If RPMs are kept steady between 1,000 and 2,000 it seems to run fine and sounds good.
- The new engine has very high oil pressure. Even when fully warmed up it seems to be running at about 85 psi.

The engine:
- Purchased the engine from a fellow bronco owner who rebuilt the engine to be basically stock with a mild, flat tappet cam (204/214, 448/472)
- The engine is a '69 351W with zero miles on the rebuilt and late 70's heads. Has a new roller timing chain installed straight up.
- Running full length headers, stock EFI truck intake and BC Broncos adapter plate mated to a passenger car style EFI upper.
- Standard Mass Air EFI system using A9M computers, Ryan McCormick's wiring harness, low pressure/high pressure pumps with a fuel accumulator, no EGR but all other sensors used including VSS and O2 sensors.

Other info:
I'm basically using the same exact fuel injection components that worked perfectly on my 302. I checked error codes and pulled codes 81, 82, 84, and 85, which is normal for an RJM harness and the same codes that I got with the 302. Even the fuel rail and injectors are the same since I widened it to fit the new manifold. It is all stock stuff using standard 19# injectors. All the components that worked fine with the old engine were re-used on the new one.

I checked for vacuum leaks but haven't found any. Timing was set with the SPOUT disconnected to 10 degrees BTDC then the SPOUT re-connected. Uses a 190 degree thermostat and when fully warmed up runs at 190 degrees. Did a compression test on each cylinder and they consistently read about 150 lbs in every one.

The person who built the engine did email me and worried that he may have installed the rings upside down, but can't be sure. I don't know if this could cause this problem or if it is something else. Maybe the engine is fine, but I'm not sure how I would test for that anyway. Any suggestions?

**Broncobowsher** · 06/16/07, 04:46 PM

Rings upside down would make it an oil burner, that's about it.
it should run better then that, I agree. If you unplug the MAF and try running it, how does it do? You have all the intake ductwork hooked up without leaks? Didn't put the MAF in backwards? HAve you watched the fuel pressure to see if it is dropping of ar anything wierd?

**Duke Nukem** · 06/16/07, 07:31 PM

MAF is in the correct way.

Actually, I think I am on to something. I checked fuses in the RJM harness and found that the 15 amp Fuse 1 had blown. I put in another fuse and it immediately blew as soon as I cranked the engine. When I removed the relay tied to Fuse 1 the engine would not crank. So it seems I have some sort of electrical short somewhere that is most likely the cause of all of this.

Is there anyone familiar with the RJM harness that knows exactly what the Fuse 1 circuit does and all the things that are affected? At this point I'm just trying to find the short. I'm not terribly skilled at this electrical stuff so any advise on how to track down the short is always appreciated.

But I feel much better now that I might be close to solving this.

**Duke Nukem** · 06/16/07, 09:52 PM

I'm getting close but also a little more frustrated. Using my ohm meter I was able to determine that the ground is coming from somewhere internal to the efi harness itself. That eliminates any involvement by the rats nest under my dash. For giggles I swapped out the IAB sensor and suddenly it stopped blowing fuses and ran perfectly. I thought I had miraculously fixed it so I reattached the relays to the fender where they have always been and got everything ready to take the truck for a spin. But when I started it back up it was the same problem with blowing fuses and running bad. Its almost as if moving the harness around temporarily fixed the short. It is puzzling since the harness was working fine for the past year and the wiring is very new. I'm perplexed at where this short is occuring.

Ryan · 06/17/07, 02:28 AM

Where is the EFI Harness alternator lead #26? Is is shorted out?

**Duke Nukem** · 06/17/07, 11:25 AM

Hi Ryan. You customized my harness to incorporate the 3G alternator harness. For me #26 includes the two plugs plus the yellow/white wire that attach to the 3G. Everything is connected as before and the battery is reading around 12V with the engine off and close to 14V while running. I did switch to new brackets since the new engine is running a serpentine setup instead of V belts, but I don't see why this would make a difference. Any ideas?

Ryan · 06/17/07, 02:52 PM

Each of my harnesses are plugged into an electrical test, which will fail any mistake, even if the O2 senors are switched. It is almost impossible for there to be an internal short right out of the box.

Without the relays the only part that gets power is the yellow wire for the computer memory pin #1. My harnesses match the diagrams on my web sites:

Un-hook the MAF sensor, O2 sensors & alternator. install a new fuse, and plug back in each item untill you find which is causing the problem.

Ryan · 06/17/07, 02:56 PM

Just in case there is a problem, what is the serial number on the computer plug?
I would like to look up its history (build date, assembly tech, ect...)

**Duke Nukem** · 06/17/07, 09:34 PM

Thanks for the help Ryan, you have definitely pointed me in the right direction. As you suggested, I disconnedted the MAF sensor, left and right HEGO sensors, and alternator. With all four disconnected the fuse did not blow. Then I plugged them back in one at a time. The MAF and HEGO sensors are OK but the fuse blows every time I plug the alternator back into the harness. So that seems to be where the ground is. Inspecting the alternator harness I don't see anything that looks wrong. It is the same alternator harness I used with no problems on the old engine. So I'm still puzzled why it shorts out with the alternator connected. I did swap out to a different 3G alternator from the one I was running but I don't see how the problem would be in the alternator itself. I do have the other 3G alternator laying around so I could plug it back in to see if that would make a difference. Or do you think the problem is in the harness itself?

You have been very helpful. I can't tell you how thrilled I am to know that it is such a simple problem and that the engine is OK. The best part is that I was able to unplug the alternator from the harness and take it for a hot lap around the block for the first time. The mass air system still needs to do some adjusting, but overall the engine ran great and had GOBS of power!

BTW, I checked the computer plug and didn't see any serial numbers. I'm assuming that you're talking about the big, wide grey plug that connects directly to the computer itself.

**Duke Nukem** · 06/19/07, 11:10 AM

OK, I've got the problem solved. Swapped out the 3G for the other 3G I had in the garage and it runs fine without blowing fuses. So that means that, for whatever reason, the other 3G alternator was the problem. Strange, especially since it was brand new. The reason why I swapped alternators was that one had a serpentine pulley and the other a v-belt pulley and I was too lazy to want to swap them. Both seem to charge just fine but one blows fuses and the other doesn't. Anyway, I'm happy that it is fixed and now the engine runs great. It figures that I change out one part and it turns out to give me all sorts of headaches.

Anyway, I'm just glad it wasn't something more serious. Thanks again for all your help Ryan. Plus I learned a lot about tracking down shorts.

Ryan · 06/19/07, 04:50 PM

Do you still want the new alternator you ordered?

**Duke Nukem** · 06/19/07, 06:41 PM

Actually I bought the alternator from BC Broncos. The funny thing is that it seems to work fine except for blowing a fuse in my efi harness. If my bronco wasn't fuel injected I probably would have never had a problem. I've owned the alternator for over a year and its just been sitting in the box. I suppose I could call Chuck to see if he would take it back, otherwise I'm not sure what I'll do with it. I just find it odd that both 3G alternators work, but one seems to pop the fuse in my harness for some reason.