Fuel pressure and knocking?

JimTMich

Member
Mar 20, 2003
161
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16
Michigan
I've been fighting knocking on my 95 stick gt since I bought it in 96. 93 octane, 10 degrees timing, replaced all the temps sensors, cleaned the maf bla bla bla. knocks above 3500rpm when it's warm or humid at WOT. So I checked the fuel pressure tonight and found 38psi without the vac hose connected to the regulator. Haynes says it should be 40-50. Does anyone know what pressure should be? Napa sells one says 39psi and an adjustable one. sounds like this isn't the problem :bang:
 
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You should always check fuel pressure with the vacuum line disconnected. Each car pulls a different amount of vacuum, which affects fuel pressure. Typically, it will pull 3-4 psi, so you probably running 42-43 psi with the line disconnected.

Have you tried knocking more timing out of it, retarding the timing to 7* or 8* BTDC? I remember you posting your problems before, and I can't believe it hasn't been solved after replacing all the temperature sensors. Have you replaced the oxygen sensors? Also, what might be a good idea if you are local: Take the car to a shop with a dyno and wideband oxygen sensors, to monitor the air:fuel ratio. Even though you are running what seems to be more than enough fuel pressure, you could have a faulty regulator, a clogged fuel rail, or some other problem that most Mustang or Ford owners don't typically encounter.

Joe
 
Thanks for the continued help.
I was testing with the car not running, so there was no vac. thus testing with the vac line removed thus 38psi disconnected.

Have not tried to take it back to 8* yet.

Have not replaced the o2's

had a shop check the engine codes. None there.

have not checked the harmonic balancer for failure.

Have not installed a 180 f thermostat. I would like to run closed loop. would a 180 put me into open loop?

I will get some water wetter to go with my freshly flushed cooling system.

Wideband o2 sensor on a dyno....sounds like fun. I think the nearest one is about 1 hour away.
Thanks
Jim
 
JimTMich said:
Thanks for the continued help.
I was testing with the car not running, so there was no vac. thus testing with the vac line removed thus 38psi disconnected.

Have not tried to take it back to 8* yet.

Have not replaced the o2's

had a shop check the engine codes. None there.

have not checked the harmonic balancer for failure.

Have not installed a 180 f thermostat. I would like to run closed loop. would a 180 put me into open loop?

I will get some water wetter to go with my freshly flushed cooling system.

Wideband o2 sensor on a dyno....sounds like fun. I think the nearest one is about 1 hour away.
Thanks
Jim
Try backing the car down to 7* or 8* BTDC and see if that helps.

The oxygen sensors could be your problem, but at 45 bucks a hit, that is a pretty expensive risk to take.

You'd know it if the balancer failed, you can rule that idea out.

Why don't you try a 160* thermostat? It would open up the passages faster and possibly keep the motor cooler.

I don't see how water wetter would help, but it's not a bad idea for the 8 bucks or however much it costs.

If at all possible, I'd try and take it to a dyno. That would (hopefully) help pinpoint the problem. Most of the time, dyno guys tend to be pretty good at figuring out what is wrong too.

Joe
 
I don't see how water wetter would help, but it's not a bad idea for the 8 bucks or however much it costs.

Water wetter not only helps lubricate and prevent corrosion in the cooling system but it also helps keep temperatures down, which theoretically could help with detonation.
 
Another possibility is that the fuel filter needs replacing or the fuel pump is getting weak. Have you tried running some good fuel system cleaner through it? The fuel system can get "gunked" up from time to time and it never hurts to run some cleaner to see if it helps. Sometimes it does, sometimes it doesn't.

Also, check the plugs to see what they look like. It also might not hurt to run a plug thats a heat range colder.
 
FEESER 94 COBRA said:
I see you have listed in your sig. that you have a cam could it be tha you need a chip or tweecer somthing of that sort? just thought i would throw an idea out there :shrug:

No, he is stock. The starter of the thread is JimTMich.

When you go to WOT, the computer immediately goes to open loop, disregarding the O2 sensors. Thus, they are not your problem.

Do a search on pinging on this (94-95) forum. There is tons of info related to this problem. Different people have found different fixes.

I am in the same boat you are in.
Scott
 
Mrgreen94gt said:
Another possibility is that the fuel filter needs replacing or the fuel pump is getting weak. Have you tried running some good fuel system cleaner through it? The fuel system can get "gunked" up from time to time and it never hurts to run some cleaner to see if it helps. Sometimes it does, sometimes it doesn't.

Also, check the plugs to see what they look like. It also might not hurt to run a plug thats a heat range colder.

new fuel filter already installed

I have been running quality premium gas with the detergents in it for years now, but I add some gumout or sea foam occationally as well.

Just replaced the plugs. They look typical white, deffinately not good if it was a 2 stroke jet ski, don't know about cars though. A cooler heat range is a good idea.
 
OK, I guess I need to look at the plugs a little closer. I'll do that when I pul them to put the AP24 plugs in.(one heat range cooler)

I removed the 192* tstat last night and put in a 160* tstat and a bottle of red line water wetter. under normal cruse it used to run at O of [NORMAL] now it runs at [. The fan of corse doesn't kick on until where it used to, so we'll see how much this helps. Now I need some heat for testing. only 54* this morning on the way to work.

Plugs tonight and as a last resort, timing retard :nonono: I hate to lose any power and don't mind running 93 octane, just don't want to trash my motor with pinging.
 
I'm having the same problem only my car is every so slightly modified. I've replaced MAFs twice, ECT, ACT, plugs, wires, fuel filter, decreased timing, I use 93 octane fuel. I bought a cobra waterpump pulley about a month ago. That is getting drilled out now to fit. I'm going to install that and a fan switch. I think that will help because like you, it doesnt ping until i hit 190-195 F. If i can get my car to run below that slightly, I think it will be fine. I'm also planning on replacing the stock fuel pump soon. I'm thinking that may help a bit as well.
 
Also, if you decrease timing and it stops the pinging, you wont be losing power. You'll be gaining power. If you get rid of the pinging you are increasing how efficient the engine runs and it will be much stronger. More timing doesnt always mean more power.
 
Still pinging.

O.K. should I put in cooler heat range plugs or retard the timing next?

Where do you get a fan switch or how do you modify the car to make the fan run earlier. When it's hot out with the cooler 160 tstat, it still run at the same temp it did before. Must be the thermostat stays open and the computer controlled fan controls engine temp. So it there anything short of a tweecr to adjust that?

Yes, I agree with you on the timing retard and pinging issue, but when it's cooler out and not pinging it makes nice power and will make less with retarted timing.
 
You can either do a search on "manual fan switch" to see how to make what I'm making or you can buy a fan switch from Pep Boys that will automatically turn on the fan for you at a desired temperature. Do not buy a $500 tuning device just to turn your fan on sooner. You will be wasting your money on a bone stock stang. People will tell you to anyways just to try and "sell" the Tweecer or whatever they use but its not worth it. If you must, just get a chip to do the same thing. Much cheaper. Just remember though, that if you do get the car to run cooler and the pinging goes away, unless you are currently overheating, this is only a bandaid fix. If your car is not currently overheating and cooling it down fixes the pinging, then the cause of the pinging still exists which means you could still have a bad sensor, fuel pump, ect, ect somewhere.

The thermostat by itself is not going to do much. As long as your fans are kicking on at 210 or so, the car will rarely see 160 or 180. So you are pretty much keeping the tstat open all the time during normal driving conditions. You need the airflow as well to get it even close to that low. Even then, you wont get it to 160 and shouldnt get it to 160. You should be running a 180 tstat at the lowest. Some people will even tell you that 180 is too low.

And I know you still think that retarding timing is going to cut HP but it wont if the extra timing is killing your performance to begin with. You can run 30 degrees of timing but the optimal setting for your car may be 10 degrees. That may just be where your car wants to run. Anything more and it may ping and create LESS horsepower. You are only going to get HP where the car will run properly. Sometimes you have to decrease timing in order to pick up HP. Gaining HP isnt really about having the most of this, the biggest that and the highest flowing this. Its about having everything optimized for what is best for what you currently have. On a stock car, you dont need much timing.
 
When you check your plugs, it would be a good idea to check the gap in them as well. When I start detonating on the juice, I know my plugs are starting to wear, which means the gap is opening up. Just something to consider.... :shrug:
 
One other thing you might want to try is to see if you can narrow it down to if all cylinders are pinging or just one. After putting the exploder motor in my car and breaking piston #8 in half, we found that there was a vacuum leak on the upper intake which was leaning that cylinder and causing just it to have detonation.