Bronco 5.0 speed density EFI mystery

[63A]

I converted my 72 Bronco (302-C4) to EFI about 10 years ago using the complete EFI from an 89 Crown Vic. The engine itself is an early 80 stock 302 with a mild truck cam and headers. The engine has been running great for the last 15 years with a few minor issues along the way (fuel pump failed, alternator, etc) but overall very reliable. I had 2 pumps - low pressure pump at tank and high pressure pump near engine but switched to sigle high pressure pump at tank in the course of trying to solve the current mystery.

Approx 3 weeks ago, my Bronco started hesitating when throttle was applied under load and would gradually get worse until it would not rev at all and would eventually die and not restart. I have a fuel pressure gauge in my dash that showed good pressure but based on previous issues, I tried replacing the low pressure pump which had been an issue a few years before. That didn't make a difference and I borrowed a diagnostic fuel pressure gauge to confirm mt dash gauge and fuel pressure is consistent 34 at idle and approx 44 when vacuum is removed from the FPR. I also put in new plugs for good measure and checked vacuum (17 at idle).

After a few days, I drove it again today and it ran great for about 5 mins- I drove around the block and got on some open road where I could test throttle response. All was working great but I had to come home when it started raining (no top on Bronco at the moment). After it stopped raining, I took it out again and all semed good for the first mile and then it started hesitating when throttle was applied. I think I am getting a very lean condition based on the sputter and backfire. Fuel pressure is still showing consistent around 34. By the time I got back home, I could barely get it to go up my driveway. It would idle fine and pull OK with very light throttle but anything more than light throttle caused it to stumble.

I've also checked KOEO codes and nothing so far. I'll check again later today but haven't seen any flashes of the CEL which I think would accompany a code.

Eng temp is ateady at approx 185 on these short trips (gauge is wired direct to sender- notreading from Eng computer)

I think I have a fuel problem but fuel pressure seems to be good and consistent.

I welcom any ideas to check- I'm stumped. Thanks

 

[oldiron]

Vacuum leak at a ported vacuum source? Would run fine until the engine came up to temp and the vacuum switch opened, introducing the leak/lean condition.
Greg

 

[Broncobowsher]

What's fuel pressure when driving and you have the problem? Fuel filter?

 

[63A]

Assuming fuel pressure gauge is accurate, fuel pressure while driving is approx 34 and doesn't vary much except to rise when throttle is applied quickly (as it should do). Fuel filter is new- don't think its a volume problem. I also assume lack of fuel volume would cause low pressure at the rail which doesn't appear to be happening.

I'll run it for a while tomorrow and check vacuum after the engine is completely warmed up to see if I have a leak in the ported vacuum.

I'm thinking I may need to chock the wheels so I can check fuel pressure and vacuum with the engine under load. If I don't post again, its because I managed to run myself over.

 

[DirtDonk]

I've seen MAP sensors fail oddly before and create even more odd symptoms.
It's possible this is an issue. But it's one of those things that is a shame to just throw parts at, so maybe someone local can test it for you?
There again is the dilemma though. If you don't know someone and have to go to a shop to test it, it's likely the test alone will cost more than just throwing a new sensor at it!
I hate the new math!

They can be tested on an oscilloscope if you know someone that has one. With a given in put they give out a nice even wave pattern, but you get an erratic or misshapen one when they're failing.

Good luck. But I would certainly check for vacuum leaks. There are a lot of ports on a Ford engine like yours. And most of the vacuum caps we get these days are total crap.
Most of yours are full vacuum though, so you would think they would act up all the time. But maybe the heat thing is what's going on here still.

Paul

 

[63A]

MAP sensor was only $17 so I replaced it - no change. I also took the Ign Control Module to Autozone to test it on the Wells machine- tested good. Retested about 6 times- enough to get the module hot.

My initial diagnosis was fuel issue but when I drove it last night, it seems more like an ignition problem. The engine idles and revs fine without load (park or neutral) but stumbles badly under load. More throttle causes more stumble. Very slow throttle application was only way I could could get it to pull. Going up my driveway was a real challenge. Fuel pressure appears to be steady around 35. When the engine does stumble, tach does drop rpm - big drop when throttle is applied quickly.

Plugs are new, cap and rotor look good, ign module checks good, new MAP sensor, no trouble codes.

I am going to try increasing fuel pressure by driving it without the vacuum connected to the FPR. That should keep fuel pressure around 45 at all times - will see if that makes any difference.

 

[BanditBronco]

I am going to toss out that it could be an injector issue, I have had two random ones fail at the same time which caused similar issues to yours. It would idle decent still but wouldn't move out of its own way and popped and backfired a little. Fuel pressures always read just fine, you can try checking with a noid light but that won't show a bad injector just proves the computer is firing injectors. You can use a stethoscope on each injector to hear if it is actually spraying or not.

 

[63A]

At this point, I'll consider any possibility. I'll try to check as many injectors as I can (some may be hard to reach under plenum) with my stethoscope.

I'm still baffled by the difference between load and no load performance. I still don't see how a fuel issue would change depending on the load or no load placed on the engine but the stumble definitely feels like a fuel issue.

 

[Timmy390]

Your not running a CAT are you?

I had one plug up (not on my bronco) and it was doing most of what you seem to be experiencing.

Fine with no load......but was spitting and sputtering and misfiring. It got worse and worse till it wouldn't go but 30 MPH but was find when at idle and when in N and reving.

Maybe check the muffler? I had kind of the same thing with one of the Samuri's I bought. The muffler was full of nuts and shells from mice......Would run fine for a bit then start choking.....

Tim

 

[63A]

Thanks for the tip. I haven't checked anything on the exhaust side. I have headers and duals all the way to exit behind the rear wheels so it will be easy to check if both sides are clear.

 

[63A]

Forgot to mention- no cats - just 2.25 pipe through shorty mufflers

 

[Viperwolf1]

I'm still baffled by the difference between load and no load performance. I still don't see how a fuel issue would change depending on the load or no load placed on the engine but the stumble definitely feels like a fuel issue.

The amount of fuel required to generate 10 horsepower at idle is roughly 1/20th the amount of fuel required to generate 200 horsepower at highway speeds. It sure sounds like a fuel problem to me. If the pressure is good it's got to be injector or ECM trouble (not demanding enough fuel).

ECMs of that era are notorious for capacitor failure. Do you have a spare or have you opened it up to look inside?

 

[63A]

Thanks for the info. Unfortunately, I don't have a spare ECM but sounds like a good excuse to go to pick n pull.

If I open up the ECM, is there something in particular I should look for?

 

[Viperwolf1]

Thanks for the info. Unfortunately, I don't have a spare ECM but sounds like a good excuse to go to pick n pull.

If I open up the ECM, is there something in particular I should look for?

Look for leaking capacitors. Discoloration under the blue things.

 

[63A]

Thanks. I'll try to open the ECM tonight and take a look. I assume the capacitor to board should be silver/solder colored versus the yellow/green in the pics?

 

[Sporto]

I had similar issues a few years ago, and it was a loose connector/wire at my fuel pump relay circuit. Created intermittent fuel delivery issues and variances in pressure. CEL came on when pumps weren't running. It took me a while to figure it out.

Also check to see if FPR is still working. There are ways to diagnose those as well, and they can lead to a lot of quirky behavior.

Good luck with your search.

Rich

 

[63A]

I removed the computer and opened it up. I can't see any discoloration around the capacitor but may try another computer from pick-n-pull just to eliminate another potential problem.

Before removing the computer, I did drive it with the FPR vacuum disconnected so fuel pressure was staying at around 44. Didn't help any

 

[ransil]

I removed the computer and opened it up. I can't see any discoloration around the capacitor but may try another computer from pick-n-pull just to eliminate another potential problem.

Before removing the computer, I did drive it with the FPR vacuum disconnected so fuel pressure was staying at around 44. Didn't help any

is that pressure measured or guessing?

 

[63A]

I have a fuel pressure gauge in the dash and I confirmed the gauge is correct using a handheld fuel pressure gauge under hood.

 

[BanditBronco]

check injectors yet? I am almost to the point I would bet someone it is an injector issue. Start pulling spark plug wires a cylinder at a time, if you don't hear a change in idle than that cylinder probably isn't firing. If it starts misfiring than that cylinder is good. My guess is you will find one maybe two that don't make a difference. I know what you are thinking, if one or two cylinders have bad injectors wouldn't it be misfiring at idle already... not necessarily. I have seen maf motors idle just fine, not sure about speed density, I would imagine they might idle better yet than a maf set up just because it isn't smart enough to know the difference.