I’m sure this has been an issue if someone’s before, but haven’t been able to find it. My 1991 Z/28 5.7 has been through it since I got it almost a year ago. When I first got it, and the car was warmed up, it would stutter and the fuel pump would groan and it would shut off randomly. This was due to a clogged fuel filter. Fast forward 8 months. This stuttering/jerking comes back without the groaning or shutting off but after a fuel tank clean and new fuel pump and filter. I have also had new MAP sensor put in. The car does not currently have a thermostat due to Georgia heat. So currently, when the car is at about 150-170 degrees, throttle response is borderline dangerous. A quick push of the gas causes it to almost choke itself out and jerk violently. Car also has a very odd idle, Will lope around 400 RPMs then go back up to about 1000. A shop told me these things were due to low ohm readings from the injectors, yet the seven I tested myself, read 13-14 ohms, which from what I read is in range. I have also unplugged the throttle position censor to see if the car would idle better or have better throttle response, to no avail. The motor is completely stock if that helps and does not throw any codes. I know this is a lot of info at once, but I really miss driving this car and hope I can fix it without blowing the money I have saved for college, which I start in a week.

 

What’s your fuel pressure at ?
thats a lower ohm reading for injectors. Are these the original multecs ?

 

As far as I know they are original, I have not checked the fuel pressure. I find it odd that all of them read within .5 ohms of each other yet they’re low. Maybe it was 14-14.5,I’ll have to double check, but the hesitation started so abruptly it’s so odd. The battery died out of the blue, got it jumped, then 30 miles later the hesitation came out of nowhere.

 

You need to check them both warm and cold.
When you check your fuel pressure, do a leak down test as well. I did mine where I recorded the pressure every minute for the first 5 minutes then every 5 minutes up until 30. Checked KOEO pressure, Pressure at idle with Vac on and Pressure at idle with Vac off.
Have you investigated grounding quality?
Most obvious here, have you performed the IAC reset and minimum idle setting? Along with verifying your base timing is correct? Never hurts to do little maintenance stuff to eliminate any easy stuff.
Do you have a way to see ECM data? How are your BLMs? Any other odd readings? How about bad information coming from your CTS?
New MAP senor.... have you unplugged it to see if anything change? New doesn't mean verified good.
Is you IAC housing clean?

You should really run a thermostat. I have live in GA, all my cars had them. Never had an issue with summer heat. If you can't run a thermostat and not overheat, there are other issues.

 

Sorry, posted in wrong thread. Hereby deleted

 

First of all put a thermostat in there. Either a 195(stock) or a 180. Not a 160. Thermostats only make sure the engine can warm up. They do not affect the ability of an engine to stay cool. Before you install it put it in a pot of water, heat it up and measure the temp. At around the thermostats temp, it should open. Put it in some colder water and you will see it close up. A higher flow thermostat would only help your cooling system (but a stock one should be fine as well). Don't buy the cheapest ones.

If your engine cannot get to operating temperature, it can act funny.

To check for vacuum leaks, you can use propane coming out of an unlit propane torch along all the manifold seals, hose areas etc. If the engine's idle jumps, you have found a leak.

Your fuel pressure regulator might also be messed up. You need to put a gauge on the fuel rail test port and check pressure at idle with the vacuum hose to the pressure regulator connected then disconnected. The pressure should rise substantially when you disconnect the vacuum hose (this simulate the throttle going wide open and no vacuum in the manifold)

 

Many don't believe this, but tip-in hesitation problems and idle issues significantly point to bad injectors. If original Multecs, they're shot. Ohms can be spot on, but that does not mean they are performing properly, especially at low RPMs, Been there, done that. Replaced injectors and problem solved!

 

I totally believe this. If the injectors are not providing accurate fueling due to slow openings or hanging open it is going to make things run poorly. I am currently working on dialing in my AE with EBL and it is very important to throttle response and the small extra PW that it brings in needs accurately operating injectors. Too little, lean and hesitation. Too much, rich and bog.