Engine hiccup

I own a 90 2+2 1.6 SOHC Geo storm which has been a great little car but the engine has started to hiccup, it sounds like every couple of seconds it missfires or something and the engine will give a small shudder.I have recently replaced the timing belt,plugs, wires,cap,rotor,ignition module,ignition coil,pickup coil,air and fuel filters,completely checked the vacuum lines and the compression checks out OK.It also seems to have lost a bit of oomph going uphill. I have set the timing and it is OK as well but as it is a pre 94 model I am told that a diagnostic would not tell me much.Do I have any hope of finding out what on earth is going on or how to fix it? Thanks Vince.

Check the fuel pressure - both at idle and under load.

The fuel pressure checked out OK,so I put in new MAP,O2 AND TPS sensors and it still revs up and down and hiccups,but If I unplug the MAP sensor the engine runs normally,what does this mean and can I safely drive it with the MAP unplugged.

Did you run injector cleaner through the system? If you have done all the other stuff a new injector might be in order. On my six engines I did not bother to find a bad multi point injector. They were 60 each so I replaced them all. At shop labor to find a bad injector plus shop markup on parts if i had more than one bad or marginal I save money by just replaceing them all. You will have serious issues if you leave the map unplugged. Do you know if you cat converter is really ok? That can cause back pressure problems and kill performance. If the cat has more than 140k on it I would consider a replacement or other fix for it.

I’m no expert but unplugging the MAP probably makes it run richer than it should but this covers up the fact that you need new fuel injectors, which is about the only thing you haven’t replaced.

I was really hoping you would not say that,I have been dreading doing the injectors in this car as it is a nightmare to get in the back of this engine and get the plenum off,I dont suppose a noid test would tell me anything other than if one had completely failed? I have listened to each with a stethescope and all have the same note and cadence ,or am I just fooling myself. Vince