Still having some issues but gaining on it. On my '93 5.0L, I don't have that deceleration feeling anymore. When I let off the throttle, it just seems to coast and coast. I used to get that strong deceleration feeling after a second to two when letting off the throttle. What causes that higher decel? I always thought it was Injector Cutout. If so, what tells the computer to do that?

I am stuck between IAC and the injector cut out.

I have no codes, timing at 12 deg. with <5 deg. of play in the chain. Timing check is good, with spout jumper in. I have been trouble shooting a different problem, and have replaced the TPS, MAP, O2, ACT, cap, rotor, cables, FPR. Truck accelerates good, gets flat at cruise, and on decelerate, just coasts.

Idle is slow to return to base at times, and can idle high at times. Cleaned Idle Air Bypass, but not convinced that it is working correctly.

Any thoughts?

EEC-IV turns the fuel injectors OFF any time the following conditions are all met:

ECT shows temp. over 140 degrees.
TPS shows closed throttle.
VSS shows vehicle is moving.
RPM is over 1500.

in addition to shutting the injectors off, the computer also advances the timing in an attempt to slow the engine even more (using trace amounts of air/fuel mixture to push against the rising piston).

i dont know about you guys, but i can easily tell when my PCM goes into and out of the deceleration strategy...the exhaust note is entirely different, and i can feel the deceleration effects. And now I can't..

 

The only cut-out is the IAC; it adjust idle speed by changing the amount of time it stays open, allowing air to get past the throttle. The EEC computer then adjusts fuel through the injectors. You can try unplugging the IAC to see if that changes the idle/decel characteristics. For emission control purposes, it slows down the decel when you suddenly close the throttle.

Your ECT seems low. The EEC needs the engine to be running at 192F or above to properly control the fuel in feedback. Do you know the condition of your thermostat? If the EEC thinks the engine never warms up, it will operate in the cold, or warm-up, mode, and could do what you're seeing.

 

Those conditions match what I experience in my truck too for the decel fuel cut. I wouldn't suspect the VSS (yet) if your speedometer still works, and the computer has RPM signal if the engine runs, so that leaves TPS and engine temperature. Have you checked idle (closed) TPS voltage? If the closed voltage is too high, the engine will not idle at the correct speed and could be the issue with the decel fuel cut. Did you reset the computer after replacing all those sensors and have you checked for trouble codes recently? You might want to check the ECT sensor for proper operation and also verify that the engine is reaching operating temperature.

 

That isn't my temp, but a reference temp that I was reading that the EEC has to have as a minimum. I have a fresh 195 thermostat installed.

I will try that tonight while driving home from work. I will pop the IAC cable wire off and see how it drives. I have been suspecting that, but it is intermittent. Some days it works as required, others, I get funny idle speeds, (too high) and I lack that hard deceleration that I used to get. I also get some really weird cruise issues..

 

I did, TPS read .9 at idle, and is smooth through the range. It is about 2 weeks old. I did reset, and scanner shows no codes. No speedo issues.

I did not change the coolant temp sensor. I didn't look at the wiring, and I probably should have. Is the Gauge sender, the same as the EEC sensor? The dash seems to read well, and it responds pretty quick, but with no numbers on the gauge, it could read low and I wouldn't know it. It is usually about 1/4" into the "normal" range. Gauge against Gauge, who knows what that is..

I will try and get a measure with temp gun at the sender, and try and see what I am running and compare the ohms on the sender and see if it is close.

I will try the IAC troubleshooting too.

 

The EEC temp sensor has 2 wires, and is different from the gauge temp sensor, which has 1 wire. They're both installed near the front of the intake manifold, and they're both kind of hard to get to with the fuel rail in the way.

 

well then, that might be worth a look to see what it meters out at.

I ran for a few miles home today with the IAC disconnected. Interesting driving with it unhooked... lol

When letting up on the throttle, I most definitely had much more deceleration and a lot less coasting. Between shifting gears, the engine immediately went to idle, no slow decel or high rpm. I believe that it is leaking by, or not managing fast enough.

I did find it interesting, that I drove it cold for a few miles on the highway, up to temp. and I never did receive a CEL. I thought for sure it would throw one with the IAC unhooked.