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Discussion Starter #1
Hi all, I finally got a new motor in my '95 Bronco and now I have an issue. First off, I have a standard bore 5.8 engine with flat top pistons, Comp Cam 35-349-8, GT40 heads, long tube headers into single 3 inch exhaust. I have replaced ALL sensors with the exception of MAP, IAC and TPS. My issue is that the engine runs great with the exception of a surging idle around 200 rpm difference (600-800). The surging I can live with but as soon as I touch the gas pedal with my finger and BARELY move it she will die. I can floor it and it stumbles a little then works fine but it will die as soon as I let off the throttle. It starts right back up and idles as stated above. I have tested both the IAC and TPS and they both are within the specs. Throttle body and IAC have also been cleaned. Timing is at 10* and plugs are gapped at .050 or whatever the sixlitre tune up called for. I know the timing is supposed to be set at 14* with the plugs gapped that wide but is that an issue? I did use the MSD coil that puts out 45,000 volts and yes I used Motorcraft Racing 9mm plug wires and Motorcraft cooper plugs. I have not replaced the fuel filter or checked fuel pressure but that is on the to do list for tomorrow, I did however install a new fuel pump. I have also replaced all the vacuum lines and checked for leaks with none found. I am also checking to see what kind of vacuum I have as soon as I get my gauge back from my friend. What am I missing? Where do I go from here?

Thanks
Scott
 

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Discussion Starter #3
Really sounds like a vacuum issue to me. Can you
Really sounds like a vacuum issue to me. Can you put it in gear?
Yes, drives just fine in gear and idles just fine in gear. Always wants to die as soon as I press the pedal but fine otherwise. Kind of like anything below or above 800 rpm is fine but wants to quit at 800. Not exact rpm but used as an example
 

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Discussion Starter #4
Ok, i got the fuel filter changed and man what a pain,. I checked fuel pressure at the rail and I have 40 psi with vacuum line removed and 32 with it connected and both readings were at idle. Weird issue is that once I shut the engine down the pressure drops to zero within 30 minutes. Possible injector leakage but i doubt it as they are new. What else would cause this? I also pulled a vacuum of 14-15 at idle, is this good?

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You said, "I touch the gas pedal with my finger and BARELY move it she will die. " The very first thing to move when you push on the gas pedal is the TPS (and the throttle plates, of course). The MAP sensor doesn't change frequency until the manifold vacuum changes (which take a little time, and the vacuum isn't going to decrease very much by just barely touching the pedal). The IAC reacts to changes in idle speed and engine load, and is controlled by the PCM.
TPS's usually wear out at the idle end of their resistor range. The engine (and the TPS) spends more time at low throttle opening than at any other position. If you have an old analog meter, check the resistance of the TPS when you "barely" touch the pedal (engine off of course). Digital meters have a tendency to change range and not give you a real good reading, especially if the TPS goes open for a fraction of a degree of rotation. If you find the TPS is faulty, try and get a Motorcraft.
 

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Discussion Starter #6
Thanks Mikey, that makes since. I did check the TPS but with a digital meter now to find an analog one. The TPS that is currently on it looks to be a Motorcraft part according to the wire colors. Could be the original one, who knows.
 

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Discussion Starter #8
Ok, I finally found a analog meter and tested the TPS. Now dont ask me what the reading was because I can't read it. Anyway, you guys were correct in that when the throttle barely moves off idle the meter drops and then picks back up . I can even get it in the exact spot to where it drops almost to the bottom on the meter. I ordered a new Motorcraft piece and will swap that out Tuesday or Wednesday. Now my other question above about my fuel pressure and vacuum.

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Ok, i got the fuel filter changed and man what a pain,. I checked fuel pressure at the rail and I have 40 psi with vacuum line removed and 32 with it connected and both readings were at idle. Weird issue is that once I shut the engine down the pressure drops to zero within 30 minutes. Possible injector leakage but i doubt it as they are new. What else would cause this? I also pulled a vacuum of 14-15 at idle, is this good?

Thanks
The pressure dropping to zero in a half hour is not a problem. The check valves in the fuel pump are not that leakproof, and they don't have to be. Why do you think that is a problem? What other problems are you seeing that make you think you have a fuel pressure problem?
The 40 PSI with vacuum line to the fuel pressure regulator removed indicates that your pump is supplying the correct fuel pressure, the 32 PSI when vacuum line is attached at idle indicates that the FPR is doing its job.
 

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Discussion Starter #10
Thanks Mikey, I'm not saying I have a problem I just wanted to make sure if it was. Before I changed the fuel filter the gauge would only build up to around 10 very slowly and I wanted to make sure all was good afterwards.
 

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Discussion Starter #11
Actually after reading my post i did say i had a problem. I thought it was but I'm glad it isn't. I will let you know if the TPS solves my other issue.

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Discussion Starter #12
Ok, I am back for a progress report. I installed a new Motorcraft TPS today with a reading of .975 with throttle closed and a 4.36 at WOT. My issue is still present and now I have a new addition. I can hold the RPM at 1500 and I have a miss. I do not have a miss above 1800 RPM. I can feel it coming on below 1800 and more prsent at 1500 and all the way down to 1100RPM. I can hold it at any RPM above 1100 or below 1800 for a few seconds (15-20) and it will just die. I do not have a miss at idle nor above 1800. This was not present before. At idle, the RPM still hunts like before and she will still die just off idle. I have changed every sensor except the MAP sensor. Does anyone know how to test it?

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Discussion Starter #13 (Edited)
I have eliminated all vacuum lines from the engine with the exception of the MAP sensor and sprayed carb cleaner everywhere I thought could have a vacuum leak and nothing changes so no leaks. I then plug everything back up and nothing changes, she still dies as soon as you touch the throttle. I did connect a vacuum gauge and at idle I get anywhere from 15 to 17 in Hg and I am 1000 ft above sea level, not sure if that's good. Checked for codes and none present except 111.
 

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Ok, I am back for a progress report. I installed a new Motorcraft TPS today with a reading of .975 with throttle closed and a 4.36 at WOT. My issue is still present and now I have a new addition. I can hold the RPM at 1500 and I have a miss. I do not have a miss above 1800 RPM. I can feel it coming on below 1800 and more prsent at 1500 and all the way down to 1100RPM. I can hold it at any RPM above 1100 or below 1800 for a few seconds (15-20) and it will just die. I do not have a miss at idle nor above 1800. This was not present before. At idle, the RPM still hunts like before and she will still die just off idle. I have changed every sensor except the MAP sensor. Does anyone know how to test it?

Thanks
The MAP sensor sends a signal to the PCM, the frequency of the signal is dependent on the vacuum the MAP sensor senses.
Disconnect and plug the vacuum line to the MAP sensor (so you don't introduce a vacuum leak), start the truck, then just touch the throttle, if it dies it's not the MAP sensor. (The only input to the MAP sensor is the manifold vacuum.)
Here's a thing on how the MAP sensor works: Manifold Absolute Pressure MAP Sensors
 

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Discussion Starter #15
Ok, I disconnected the vacuum line to the MAP sensor, plugged vacuum port on the tree and started the engine. The engine was hard to start but it did and then it proceeded to stumble before settling in at 1100 rpm. The idle never came down and it idled for a good 5 minutes then it died. I restarted the engine and with the engine idling at 1100 rpm I removed the vacuum plug and connected the MAP sensor back onto the tree and the engine idle dropped down to around 700 rpm and idles fine. Engine still dies as soon as I touch the throttle with MAP connected but not with it disconnected but the idle was already at 1100 rpm when I tried it.
 

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I had a similar issue years ago with my bronco. It would start up and idle and I could put it in gear and drive.... but if I pressed the gas it would stumble and die. If I stayed in park I could rev it up but in gear it would just stumble and die. It turned out to be the ignition control module.
 

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Discussion Starter #18
Here is a new development. I had to move the Bronco so my daughter could get out of the driveway so I thought I would cruise the neighborhood and see how everything acted. Well I was in drive going down a hill at idle, just coasting and at 6 mph the engine shut off. I can duplicate this everytime! I then decided to try it in neutral and the same results. I then unhooked the speed sensor at the rear axle and with no reading on the speedometer it did not shut off. Any ideas why this is happening? The engine still dies when you touch the throttle.
 

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If you decide to change the ICM, get a BLACK one. Preferably Motorcraft. Most parts places will try and sell you the wrong ICM, as the computer listings they use are often wrong. If the parts place tries to sell you a grey ICM, tell them you want an ICM for a 95 Mustang 302. Usually they will come back with the correct (black) ICM. Clean all the old heat sink compound off the heat sink, and apply new heat sink compound to the heatsink and the back of the ICM. Computer stores and even WalMart sell heat sink compound, it is used on Personal Computer processor chips. Arctic Silver is a brand name, but any brand should work. The ICM generates a lot of heat (that's why it sits on a big aluminum heatsink).
There is a special socket, called a Ford Ignition Module Tool, look here for what it looks like. A regular socket doesn't work. You should be able to find one at any good auto parts store, or from Amazon:
Amazon.com: Lisle 64650 Ignition Module Wrench for Ford: Automotive

The idling down hill (with the throttle closed, obviously) and then stalling sounds exactly like the problem you were seeing in your driveway. Idles OK, touch the throttle and it dies. So that problem hasn't gone away. I don't know why the VSS signal would have any bearing on the engine dying problem, unless you are experiencing some strange PCM failures (which is always possible on a 25 year old vehicle).
 

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Discussion Starter #20
I decided to back track everything today and see if I missed something and everything looks good. I then decided to unhook things to see if it made a difference and the first thing I disconnected was the power to the EGR and now it doesn't die. I connected it back and it dies. Can someone explain why it does this? I get no codes unless it is unhooked.
 
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