November 6, 2010OK, I went back and looked over the whole conversation.
From your neighbors complaining, I am going to guess that you DON`T have the stock cam anymore, or stock mufflers. Probably dumps or something - that`s the only thing I can think of that is loud enough to get the Police to your house!
So, non-stock cam may require the throttle cracked open a bit before you get to the bottom of this and have a nice steady idle. If you have a non-stock cam that is `big` (high lift and/or some lobe overlap), it might surge a little at idle no matter what. People used to like that. I still do, but not on a Speed Density controlled Mustang, where it messes up the computer. The solution in the end, again, may be to just crack the throttle open a little at idle using the set screw that is under a cover (you`ll have to drill out the cover on your car if the PO didn`t do it).
Before you get to that, you mentioned that when you press on the throttle there`s a big delay before the engine goes. That is NOT normal. A 5.0 has pretty good throttle response. So, I am going to focus on that. If I remember correctly, the Speed Density computer system that you have looks at the throttle position sensor and the BAP sensor (barometric pressure) and the RPM to figure out engine load and then decide how much gas is needed.
- If the RPM measurement was wrong, you probably wouldn`t be running at all, because it is also required for spark.
- If the barometric pressure is wrong, you`ll get too much or too little gas and unpredictable (bad) running. Hmmm.
- If the throttle position sensor is wrong, it`ll probably have a dead spot where the throttle in that position runs bad (usually the engine dies) but a little less or more throttle is OK. Does that happen?
So, I think you should find the air pressure sensor. It should have a vacuum line fitting that IS NOT CONNECTED TO ANYTHING. It`s measuring air pressure. No cap or line on it at all. I remember it being near the firewall somewhere, maybe on the passenger side. Sorry I can`t be more specific - it has been YEARS since I looked under the hood of one of these cars.
I think you should maybe measure the output of the TPS - Don`t worry about all that `is it 0.98 volts at idle` stuff you`ll find all over the net. That stuff is to make sure the computer goes to WOT when you floor it. Right now we just want to make sure the car idles, which is almost completely unrelated. Mark where the TPS is mounted now, and loosen the screws and move it a little. The computer will re-measure where `idle` is when you turn the key to turn on the car. Hmmm - you are not holding the gas to start this car, are you? Keep your foot off so the computer can `find` idle.
And, going back to RPM measurement and spark control. You never told us what your timing is. Find/buy/borrow a timing light. Take out the SPOUT connector and measure the timing at idle. It should be rock solid at 10 for the stock setting, and not more than 14 usually if somebody bumped the timing. If it`s WAY advanced, which some people end up doing when they tune by ear, your car will run all screwey. Get it in the normal range (10-14 BTDC at idle, SPOUT connector out) and then put the SPOUT connector back in.
Last - is this car throwing any codes? There may not be any `check engine light`, even if it`s in the dashboard. I forget what year that got added in all the way. If you turn the key to ON but don`t start the car, all the dash lights light up to check which bulbs are burned out. Is the check engine light on? If NOT, then you don`t have a check engine light. To read codes you will have to beg/borrow/buy a code reader for your car, and connect it to the car to test it.
I am just trying to go through all the things that could be wrong in my head, so I know I ask a ton of questions. When you check things on the car, can you let us know what you find in those areas (or even answer my questions directly, one by one?) so that I can know when to drop the subjects that aren`t the problem? Otherwise, this conversation is just shots in the dark by me... but I will keep trying! I hate to see a sick Mustang.