Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

My car wont go over 45 mph & doesnt accelerate

i have an 1992 infiniti q45 I replaced the cats some coils,fuel filter,maf sensor,spark plugs, My car had this problem in december and I replaced all that I just listed.but now it slowly has gotten worse then before.It has good gas pressure.the fuel pumps good.Theirs no check engine light on.The newest symptoms are it doesnt accelerate at all.I cant get it to go over 1800 rpms if I try it bucks or has no response.and the longer the drive the worse and worse it gets.I eventually cant go no more.It comes to a crawl to 30 mphs at 1500rpms.It has a rough idle but its not that bad.If I rev it in park I can get it to go up to 4000rpms but quickly goes done to 700 rpms and then wont go back up unless I take my foot off the gas.Sometimes it runs alittle better but not often enough to count.No one can figure it out since theirs no check engine light.Before I changed the MAF SENSOR it was blowing out alot of fumes,not oil or anitfreeze but emissions or fuel.but I changed that and now it doesnt do that anymore.And Im sick of getting ripped off.So please help me out in anyway.Has anyone had these similiar problems?Does anyone know what it could be?

How many miles on this car?? Does the CEL come on when you first turn the key on?

Old car: disconnect the battery for a minute and reconnect. If the problem goes away for a little while or for a day or two and comes back, suspect a bad computer. No warning light? I would wonder about the computer.

Define good gas pressure. What kind of PSI and at what volume?

You changed the cats but what about the muffler itself? While rare, mufflers can clog up; especially if a problem has been going on for quite a while.

159000 and the cel comes on when i first put the key in ignition.the light works it just doesnt come on when this is happening.

the mufflers were done about a year ago and then in december I had the cats done ,the fuel filter ,a coil,maf sensor,after that it ran better but not a 100%, and it slowly got to were its at doesnt have any response when i hit the gas when its at 1500rpms,it bogs down and sometimes if i hold it their it (at 1500rpm) it backfires in the intake.and before it got this bad it used to sometimes not let me go past 5000rpms when i got on it,it would get to 5000rpms and not go past like it was stuck,then other times i could get it all the way up in rpms.but now it wont go past 1500rpms and doesnt accelarate at all. and it gets worse as i try and drive it.could the old cats have clogged my mufflers and the new cats got over heated and because of that and are now shot?the exhaust putters when idling.i really cant figure this out and need some real help.

This sounds an awful lot like poor or erratic fuel supply to me. If I’m correct in that this '92 is a throttle body injection system, the injector itself is a really good possibility. That’s assuming you’ve tested the fuel line pressure and the fuel pressure regulator.

In all honesty I’d want to put an ignition analyzer on this too. An erratic ignition would not be ou o the question either. You simply cannot beat looking at the signals while trying to rev the engine to see what’s going on.

Changing parts without doing some diagnosis is unlikely to get this running right. More likely, you’ll just keep spending money until you finally give up.

It’s difficult to even offer much of a wild guess when even the basics are not provided.

To reiterate, you state the gas pressure is good so I ask again what that gas pressure is.

yes but wouldnt it not backfire in the intake if it wasnt getting enough fuel?and before i switched the maf sensor it was very rich , it had excessive emissions coming out the exhaust.and it doesnt stutter like its running outta gas more like somethings holding it back from theirs pressure and the pressure has no where to escape to.when i press the gas harder it bogs and sounds like it has no power.and at first it was an intermittent would run good sometimes but most of the time it would run crappy.and the fuel pressure was tested by a mechanic ,hes the one that said it was “good” so im not sure what the reading was.i also did the spark plugs a few months ago and when they were taken out a few had alot of carbon built up on them.i dont know if that matters but i just cant figure it out and it seems like no mechanic can tell a 100% positive answer on what it is , they always fix something on it and it runs about 80% bettter but not 100% like it should.thats why i came on here to get some help and even if someone can tell me a direction i should look into and go from there.thanks for all your replies and please keep em coming i need as much help as possible.

There’s still not enough info known to do anything other than wild guess. Low fuel pressure can cause backfiring.

About all I can do is list some things that can cause an engine to run very sluggish, maintain low RPMs, etc.
Low compression. (just how hot did those converters get? entire engines can get roasted this way)
Fuel pressure too low.
Fuel pressure regulator shot.
Clogged exhaust. (cats, muffler, some comedian stuffing a rag in the tailpipe, etc.)
Major vacuum leak.
Ignition timing that is seriously off. (usually retarded too much, and this can also overheat cats)

I know what I would do if the car were mine. I’d run a compression test and make double-sure there’s not a trashed engine problem due to overheated cats, etc.)
You always make sure that you’re not beating a dead horse right off the bat.
After that it would be a fuel pressure test, timing check, and inspection for vacuum leaks.