Hi, I am new here and I have a bit of a problem. My 89 IROC-Z with the 5.7L TPI engine is shuddering at all RPM ranges whether the car is driving or in park. It's not bad at idle, but from 2500-3500RPM it shakes a whole lot. When I first got it, it had a cold start issue, that turned out to be 3 bad fuel injectors. I replaced all 8 of them and that issue is fixed, but the rough running is still there. It also doesn't feel like it has a whole lot of power. I have the following done and it still has issues. 1. New spark plug wires. 2. New distributor cap and rotor (Replaced by the previous owner, have reciepts and inspection confirms) 3. New fuel pump and filter. (Replaced by previous owner, confirmed with 50PSI at the rail) 4. 8 new fuel injectors 5. Set the timing, though I am not sure if it's right, someone painted marks and what I assume to be the correct mark on the gauge. No engine codes either. Any ideas of where I should start looking? Going to try a compression test next to make sure I don't have a burnt valve somewhere.

Crap. I think I posted this in the wrong section. Sorry if I did, and mods please move it if I did.

well its a TPI isn't it? start with timing since you have no CEL. Timing i believe is supposed to be at 7 degrees BTDC, you have to disconnect the wire on the back of the dizzy, start her up, and spin the Dizzy till you achieve the proper timing. now the most difficult part for you is goning to be finding the stock timing marks since someone oviously marked it for some unknown reason. that where i would start

Yeah, it's a TPI, but I saw a general tech section too. On the gague, it looks like a saw blade, there are two marks, then a deep mark, then three more marks. The paint is on the deep mark, which is where I set it to. Been a long time since I have had to set timing, but I think it's correct. I am too spoiled by the computer controlling the timing.

I would assume that whoever marked it did so to set the correct timing. Most people use chalk, not paint.

recheck the fuel preasure at 2500 rpms

have the ign module checked

check the res of all injectors

check coil and pickup coil

adjust timing by ear

does the motor have a knock

any codes

how does the motor sound when it starts acting up

is it a rpm drop

is it choppy

more info is needed

Will recheck the fuel pressure. The coil looks original, so it's on my hit list. I tried setting the timing by ear, but the shudder was still there. All of the injectors were flow matched and resistance checked by the manufacturer, but I did check it myself and it's fine. No codes, no knocking. RPM's are fine, no drops at all, runs fine other than the shudder and seemingly lack of power, but I have never owned one of these, so I am not sure how it's supposed to feel when it's right. The motor even sounds normal, no missing or backfiring.

Ok, I found the info I needed in the tech section to set the timing. It's set a 6* now and the power has returned. I also swapped out the ignition coil for a new one. The shudder is lessened by 50%, but still there. I just found out that my knock sensor wires have crumbled and my engine coolant sensor is malfunctioning. Gonna fix both of those and see if that fixes the problem.

I used to have the same problem on my 88 GTA and it was the MAF.

before you start throwing parts at the car by a multimeter and do some tests

Found it. Several fuel injector connectors weren't making good contact. Changed em out and it fixed it.