Lost top end power
 

I bought my 91 LX about 5 months ago. When I first got the car it would scream. The powerband would kick in around 3000 rpms and pull hard until I would shift around 6000 rpms with no signs of letting up. You could even get the tires to break loose at the top of second. Well, over the last few months my car has lost that power and I'm not sure why. I can feel the powerband kick in at 3000, but it only pulls until around 4500 rpms and you can just feel that the power isnt there like it was. Everything on the car was put together by the guy I bought it from...bored .30 over, balanced stock crank, 10.5:1 KB pistons, ported 289 heads with 2.02, 1.60 valves, 588 Ultradyne cam, cobra upper and lower, long tube headers, etc. I thought maybe I was just getting use to the car, but I emailed the guy I bought it from and he said the car use to pull hard up to 7,000 rpms when he had it. I'm just wondering if anyone has any ideas on what could cause a power loss up top like that. I did have the car dynoed and the sheet shows power dropping off very rapidly after 5,000 rpms. I am going to check my timing tomorrow and get a fuel pressure gauge so I can check that. If anyone has any ideas please let me know. Thank you.

my first guess would be timing, then fuel pressure, and if none of those work. i would try switching mass airs with someones that you know works. i know of about 10 people that had mass airs go bad. but check the timing first, then fuel pressure. let us know what happens

I'm just hoping that I havent been running lean on fuel:) How can the mass air go bad?....the sensor inside it? There is a fuel pressure gauge under the hood. Can I check WOT fuel pressure with the car sitting and reving the motor? What should it be at? I know at idle FP is at 40. As far as checking the timing, what plug is it that I have to pull from the distributor? I went out and looked and it looks like there is only one harness of wires that plugs into my distributor. Is that the one? I'll still check timing, but I havent messed with it since I got the car so I'm doubting thats the problem.
When I first had the car, all through second gear you could feel it pull super hard like it wanted to rip the tires off the car. After awhile it felt like that pull would be there on and off. And now I just dont feel it at all. It's not like there isnt anything there, but the power and the extent of the power through higher rpms isnt. I'll check to see if I could borrow someones mass air sensor. I've got 24lb injectors but I think there are some stangers around here with the same.
Could a faulty fuel pump or injectors be a culprit? I know its got a FMS high volume fuel pump in it. Not sure how big. The guy I bought the car from said the car is good to go as far as a fuel pump and that he planned to put a 150 shot of NOS on the car, so I am thinking the fuel pump should be fine.
Hmmmm...fun stuff. I guess I'll check some things out and see what happens. Any other ideas are appreciated. Thank you!!

fuel pump could be the prob. thats why you should check the fuel pressure, and no i dont think you can check it just by revving it. i think the motor has to be under load. the mass air elements can go bad, the little elements in the mass air are heated, and when the air passes by it, it cools the sensor, and sends a voltage signal to the computer. the plug on the harness is grey and is square, you have to unplug it, inorder to check your timing. otherwise it will tellyou the timing is at like 25*. i think its right next to the distributor on the fox bodies, not sure though. one more thing about mass air sensors, when they go bad you USUALLY only notice it at WOT, it will usually bog down and fall on its face when you floor it, but runs normal under part throttle conditions. i would check your timing, then your fuel pressure first though.

In order to check the timing you need to remove the SPOUT connector. Just as described by sn95gt19. And, yes, it's located right next to the distributor - should be just lying there near the lower intake.

You can also leave the SPOUT in and check what's called 'total timing'. For example, my total timing is set to 33 degrees.

Try 12-14 degrees with the SPOUT out or 31-34 degrees with the SPOUT in.

Best way to check fuel pressure is with the engine under load. Autometer makes a nice in car electric gauge, but it's around $185. My buddy has one in his 91 coupe - very nice setup.

You also may want to run a quick code scan just to see if anything is showing up there.

Good luck,
E

When checking my timing, what do I line the line up with that is on the dampner?

The only thing I can remember doing to the car since losing power is taking the A/C off and putting an eliminator kit on it. Is there any vacuum line I could have unplugged or anything to that effect that would change things? I unhooked all lines running from the A/C unit and that canister that is bolted to the firewall. Just an idea.

7000rpm? Id check into valve float, maybe your springs are toast.

SKyler

Whats the best way to tell if your valve springs are shot? I had written in an earlier post that a more gradual push on the accelerator seems to have more pull than giving it WOT and someone had mentioned something about valve springs. How big of a job would that be to change them??...if that were the case. I checked my timing today and that is fine. Still dont have a fuel pressure gauge to check the FP yet.

I am leaning more towards valve springs....

You'll have to disassemble the heads/valves to get the springs out. Once they're out, you put them on a spring tester. It basically compresses the spring and tells you what type of pressure it is resisting at certain heights. You can always shim them to 'get' the right pressure, but I don't recommend it.

FWIW, I just finished building up my 331 and tested all the springs while they were out. Ended up that 7 out of 16 springs were out of tolerance, so I replaced all of them.

Hope this helps.
E

Okay...I just got an email from the guy I bought the car from and he says he highly doubts that its the valve springs. They had about 5k miles on them before he sold the car to me and they are K motion valve springs. So, I think I am going to check out other things. I bought a code scanner today and maybe I can find something. I'm starting to think that I may be running lean on fuel. I've been using the post search on this site and I found that with a car running lean that it will pop and back fire when you let off the gas, and my car does that quite a bit. Any time I down shift or let off the gas the exhaust pops. I thought maybe it was from an exhaust leak but anyone hear of a car doing that from a lean condition? I am really hoping that if I am running lean that I havent screwed my rings up or anything like that. Anything I should know or be aware of when using the code scanner? I'm sure I'll figure it out. Thanks for the help.