joes c4 cab

Thank you all for the help. Here’s what we did today toward diagnosing the problem:

pullled DME computer and tested throttle position sensor and it checks good on and off throttle

made a jumper into the diagnostic port to bypass the idle control valve. - no change

pulled the vacuum line on the fuel pressure regulator to check for fuel. None present. And no rich running black exhaust.

disconnected (new) oxygen sensor to let the computer run the ROW map - no change

tested power at ignition coil - all correct. Disconnected wiring from battery and tested resistance of the coil itself and it tested ok.

used inline spark tester for all plugs and all check ok

removed air box cover in case it was too restrictive. No change.

i do not have fuel pressure kit right now but I will go get it for diagnostic sake, but when we got the car we cleaned the tank by running thru 2 gallons of fuel to flush the tank, changed the fuel tank strainer, fuel pump and fuel filter. Filled with 2 gallons of fresh fuel. Also checked speed and reference sensors and all checked out.

all this started right after our smog check. The tech drove it out and it was stumbling. Even today I started it and it ran to temperature perfectly for around 10 mins before it started stumbling at idle.

right now we feel like we have tested just about everything we can test and we ordered new injectors. Mind you we initially got it running by tapping on the injectors to get it started.

I took the car out today and it stumbles at low rpm and idle pretty consistently but once you get to 3,000 rpm it runs flawlessly.

weird. When we get it solved I will write the resolution.

As always thanks for any help.

Jfrahm

What did the plugs look like? Best to check after chopping it off when the idle is acting up and checking them right away.

Did you try adding fuel (spray carb or brake cleaner) to see if it evened out the stumble?

The FPR could still be leaking under high vacuum. Or there could be a significant vacuum leak. Sounds unlikely but it's a subtle problem, if it was easy you'd have sorted it by now. Injector lower seals, intake gasket, etc. Something that opens up when the engine is warm.
Spraying water with a squirt gun can affect a leak like this as it plugs the leak for a bit, or carb cleaner gets drawn in.

Careful with replacement injectors, there are a lot of garbage fakes on the market these days.

mwc951

Dont do it!
Google searching is better bet.
Ask me how I know.