Help, engine fails to accelerate past 1/4th throttle

MustangLX-5.0

MustangLX-5.0

Founding Member
Dec 2, 2000
858
7
38
Lehigh Acres Fla.
#1
When I step on the gas a little, it revs up okay. If I go any more than 1/4th throttle, it barely speeds up. It just starts sucking in air. If I floor it, it will just sit at the same engine speed while its sucking in tons of air. If I snap the pedal fast, theres a delay and then it shoots up a little then slowly drops.

I don't know what happened. It was running perfect. I had everything set right. I had just got done taking the car out for a second test drive after I messed with the CC plates again. The car sat for about 3 hours afterwards. Then I decided I was going to try and do something about the loud buzzing noise the coil has always made. I removed the coil with the bracket still attached. Before I did this, I disconnected the battery as usual. I then put a plastic washer between the coil bracket and the strut tower ( where the screws go through). After I got it all back together, I did the usual, re-connected the battery and started it up. I let it idle about 4 - 5 minutes so the ECU could re-learn its idle etc. Noticed the buzzing noise wasn't all that loud as it used to be. I drove off. Engine was running fine. I didn't have room for hitting the gas harder than 1/4th, till I got to the main road. Thats when I noticed it. Its like its running out of gas allmost. Just sucks in air past 1/4th throttle. I checked fuel pressure and all was normal. I know my TP sensor is fine, I just tested that recently. Shut the engine off, waited a minute, then started it. Waited another minute then plugged the code scanner in. Only codes I get are the same ones that have been showing up for years. 41 & 91.

Does anyone have any clue as to what would cause this? It runs smooth. Just that it hardly does anything if I hit the gas past 1/4th the way in. The more I press it in, the more it just sucks in air.
 

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#3
Removed the plastic washers. I thought it fixed it because when I started it up, it was running great. Response was right on the dime. I let it idle till it warmed up. I can always tell when it's warmed up enough because the idle speed will drop by about 100 RPM and the timing will change. As soon as it warmed up, it went right back to the bad throttle response. I don't know what the problem is. Damn it, it seems that everytime I disconnect the battery, theres a new problem. ****in Fords...
 
#4
hey hey now dont go getn all upset with the fords =). if your tps is set right, and idles correct, and all the ignition is set up right. i had a problem like that once and it was because i had to adjust the clutch cable lol...it was slipping real bad, would rev up then drop...just one problem i had before..
 
#5
It's not a clutch .. it's an AOD. To describe the problem a little more. It's like it's not getting enough gas.. the engine continues to run maybe a little faster, but not by much, and the intake just sucks in air without it speeding up. It doesn't stall or anything.

I'll check the TPS .... again.
 
#6
MustangLX-5.0 said:
It's not a clutch .. it's an AOD. To describe the problem a little more. It's like it's not getting enough gas.. the engine continues to run maybe a little faster, but not by much, and the intake just sucks in air without it speeding up. It doesn't stall or anything.

I'll check the TPS .... again.
Click to expand...

lol i didnt see that my fault haha. ya i guess just check the tps mainly.
 
#7
Sitting here next to the car now. It's running normal.. like any good running 5.0. Fuel pressure holding steady at exactly 33 PSI. Fuel pressure goes up to 40 PSI when I push the throttle hard. But it's still not revving up hardly. Theres a big delay in the throttle. It will rev up a little when I push the throttle hard.. The intake will get loud because the engine speed hardly speeds up and the exhaust quiets down.. well because it just isnt firing up to keep the engine speed harder. Last time I did this, a huge backfire blew out the intake. I'm not doing it again afraid it may damage something. I just checked the TPS. Exactly 1.04V at idle. It will jump to over 2.5V when I rev it up... or try to. So TPS is good. Still not getting any new codes. My only guess is that something happened to the coil... or the TFI is going bad.

Anyone know what the resistance reading is on a good coil? I know it probably isn't the same as a coil within a coil pack on an SN95.
 
#9
Update. Well it turns out I was wrong about this only happening after it warms up. I just didn't notice it right away. Yes, the throttle is more snappy when I first start it up, but it still cuts out a lot when I throttle it hard.. or when I step on it more than a quarter of the way. Both in drive and in park/neutral.

I'm bummed as to what this could be. I want to test the TFI, but I dont know where to start. Every time I search for TFI testing here, all the off site links to the articles related are dead. figures..
 
#12
MustangLX-5.0 said:
Update. Well it turns out I was wrong about this only happening after it warms up. I just didn't notice it right away. Yes, the throttle is more snappy when I first start it up, but it still cuts out a lot when I throttle it hard.. or when I step on it more than a quarter of the way. Both in drive and in park/neutral.

I'm bummed as to what this could be. I want to test the TFI, but I dont know where to start. Every time I search for TFI testing here, all the off site links to the articles related are dead. figures..
Click to expand...

Take your tfi to autozone. They have equipment to bench test the module. A ford tech told me it's also a good idea to replace the pickup at the same time since the pickup will most likely be in poor shape as well.
 
#14
TPS voltage at FULL WOT is 4.5v. And the response curve is very precise.

Running out of ideas. I DOUBT it is the TFI modue because it is sparking very clean and accurately. Noted earier how I can hear the coi firing from that buzzing/ringing noise I mention earier. Well I stomped it to WOT and held it there for about 3 seconds. I can clearly hear the coil firing while the engine just sits there and sucks in air and barley speeding up.

I unplugged the MAF just for the heck of it. Engine stalled of course. Left MAF unplugged and started it back up. Stomped on it good. I coud not hold the pedal this time. Engine REALLY wanted to go... so... I thougt.. WTF.. bad MAF? I just tested that thing when the engine was running like crap a couple weeks ago. I recall getting a 68k ohm reading on pins C & D with key off. I check it now and pins C & D or pins D & B give me a reading of 3.8k ohms. Yeah.. three point 8 thousand. Just making sure you read that right. I unplugged it and tested those 2 pins again, 4K ohms. The readings should be well over 10K ohms.. should it not be? I plugged it back in, started the engine and measured for voltage at C & D. 1.2v . And it increased up to 3V when I would rev the engine up .. or try to. Is the MAF bad or not? The V readings seem ok, but the resistance check appears too low.. but IDK..

I'l get the TFI checked as soon as I can get it up to a parts store. Only had time at night to look at it.
 
#15
MustangLX-5.0 said:
When I step on the gas a little, it revs up okay. If I go any more than 1/4th throttle, it barely speeds up. It just starts sucking in air. If I floor it, it will just sit at the same engine speed while its sucking in tons of air. If I snap the pedal fast, theres a delay and then it shoots up a little then slowly drops.

I don't know what happened. It was running perfect. I had everything set right. I had just got done taking the car out for a second test drive after I messed with the CC plates again. The car sat for about 3 hours afterwards. Then I decided I was going to try and do something about the loud buzzing noise the coil has always made. I removed the coil with the bracket still attached. Before I did this, I disconnected the battery as usual. I then put a plastic washer between the coil bracket and the strut tower ( where the screws go through). After I got it all back together, I did the usual, re-connected the battery and started it up. I let it idle about 4 - 5 minutes so the ECU could re-learn its idle etc. Noticed the buzzing noise wasn't all that loud as it used to be. I drove off. Engine was running fine. I didn't have room for hitting the gas harder than 1/4th, till I got to the main road. Thats when I noticed it. Its like its running out of gas allmost. Just sucks in air past 1/4th throttle. I checked fuel pressure and all was normal. I know my TP sensor is fine, I just tested that recently. Shut the engine off, waited a minute, then started it. Waited another minute then plugged the code scanner in. Only codes I get are the same ones that have been showing up for years. 41 & 91.

Does anyone have any clue as to what would cause this? It runs smooth. Just that it hardly does anything if I hit the gas past 1/4th the way in. The more I press it in, the more it just sucks in air.
Click to expand...

Are you sure you didn't stomp the gas real hard and broke the pin that hold the acceleration cable to the throttle body? If you did, that's it.
 
#16
Just for the hell of it. Look in the throttle body while you have someone floor the pedal and see how far it opens. Just to see how far the blade is actually opening from the gas pedal its self
 
#17
Throtte plate opens 90 degrees (full) . It's not that. I'm willing to bet almost 100% that the cause of this is the MAF sensor. The problem goes away completly if I unplug the MAF sensor. I just reaized .. once again that the problem will NOT always present itself when the engine is cold. A few times it did, but on the last 2 starts where the engine had not been started for at least a full 24 hours, the problem was non-existent. As it got near 100 degrees in coolant temp, the problem started sllowly popping up. When it was running for a few minutes, the problem had gotten worse. If I shut the engine off and let it sit a couple hours then started it, the problem was there immediately. Its not a bad temp sensor. I just repalaced it, and even checked it recently. It's working fine. I'm thinking the MAF is burning out or something. The resistance in pins C & D and pins D & B are way too low. They are reading 3.9K ohms with it unplugged. 3.6K ohms with it plugged in. The harness test gave me good results. 12V at pin A and any ground incuding MAF ground. Harness connectors for C & D (Sig & Return) not connected to MAf gave me over 150K ohms. So there is definately NOT a short to ground in the wiring. The MAF itsef appears to be shorting out.. with such a limited range (3.6K ohms) , it could very we explain why it's not acceerating past a quarter throttle. But I need another MAF sensor. JY didn't have one to sell that worked. Will see what LRS prices them at.
 
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