joes c4 cab

Many of you know my son and I are working on a project car to resell. It was running great, and we recently got it smogged and registered in CA for the first time in years. Now that’s it’s registered we want to drive it more and shake it down for any remaining issues.

then this started. The car seemed like it was out of gas but since filling up it hasn’t gone away. Starts right up immediately after sitting a few days, then after driving a bit, it starts to idle like this. Then it doesn’t go away.

no new vacuum leaks that I can hear and holding a screwdriver to each injector seems like each is clicking.

I should probably start with fuel pressure, but does anyone know what causes this stumble?

thanks.

https://youtube.com/shorts/4ahN7DNRxs4?si=lFxDgq9swh_Ngh3s

joes c4 cab

Tested throttle position sensor (TPS) and it tested good.

we are thinking one of the injectors is fouled. I’ll report back.

mj951

Wouldn't hurt to have them cleaned, flow tested and rebuilt.
https://witchhunter.com/injectorserv1.php

joes c4 cab

Just tried to jumper the idle control valve with the diagnostic port. Car will not start. Removed and car will not start. Feels like a fuel issue.

im thinking fuel pressure regulator now. I’ll try the vacuum line test tomorrow and see if there is fuel in there.

EDIT - I just realized I have not reconnected the DME yet after testing the idle control valve. Uggh.

Tired and frustrated so I’ll start again tomorrow.

Jfrahm

Looking at the spark plugs often tells you a lot, particularly if you cut off the ignition when it's stumbling as you might find the plugs to be a bit wet, black with carbon, etc.
Yes the FPR might be falling apart, many of them seem to be failing due to age and fuel additives these days.

A DME relay jumper is good for testing fuel related issues bot to eliminate a failing relay causing an intermittent fuel pump but also so you can pressurize the fuel system without cranking/running and having a lot of sparks handy to ignite any spilled fuel (the distributor cap is full of nice fat sparks and right by the fuel rail).

Cameronjames

I’ve thought about cleaning them myself but it’s a delicate situation so if you have an extra $100 bill laying around I would consider going this route. Thank you sir for showing the water 🤙

walfreyydo

Follow the steps. Test before replacing parts...
https://www.clarks-garage.com/shop-manual/ts-01.htm

mwc951

Sadly...WitchHunter is no longer doing injectors.

mj951

Well dang.
Good to know, thanks.

Just saw Gordon's note on his site.
Looks like he's recommending his friend Curt out of Wichita.
https://www.advancedinjector.com/

Z3M&968

I read this thread twice before replying. Yet, not being a genius, I may have missed something anyway. I would begin with a fuel pressure gauge T"d into the rail. Then, if low pressure presented, I would look at filter and strainer, in that order. The filters on 911//924S/944//968/986/987 and so forth cars are ridiculously oversized, some lasting 30-40 years as long as the car was regularly driven - not necessarily for long drives, but not left in a barn to rot. The strainers seem more often to be to blame.

If adequate fuel supply presents, I would question observations during the diagnostic, and as a result, follow the classical diagnostic order where ignition and electrical systems precede fuel systems. Crank, Timing, Compression, Ignition, Fuel, and finally Combustion with resultant power output, hopefully in one direction!

Easy first electrical check. Check your grounds and other MAIN power terminations, provided you've exhausted fuel supply problems as much as sunk time permits. Check voltage drops across mains to ground and to starter. Start the car and measure voltage drop from alternator to battery - it's a series. Main battery cable to starter, then alt/start harness from starter to alternator with a T hooking up behind the brake booster.

As I acquire data, I will offer more.

Cameronjames

Any tricks or tips for removing only the strainer? Everyone says it’s almost guaranteed to break away with threaded insert

Jfrahm

This IMO is difficult and not accessible to the average bear. You either have to bodge something or get a particular fitting for the rail, plus get the cap off the rail without cracking something or losing the sealing ball. And a leak can cause an engine fire.
Better IMO to see if it clears up with a little extra fuel and if so, start to look into fuel delivery back towards the pump. Or maybe at the return line fitting?
If it's lack of fuel delivery, spraying a little carb cleaner into the TB might settle it down and that would be cheap and easy.

The injectors probably should be cleaned at some point but what is it about a dirty injector that would cause a rough idle "after driving a bit" ?
If fuel delivery is an issue, more likely it's got flakes of old gas in the tank or something like that.

Jfrahm

Hose clamp around the tank barb.

Just watched the video, wondering about the O2 sensor. After warmup if it's getting into closed loop bad data from the O2 sensor can cause idle problems. Or maybe it has a significant vacuum leak that is masked during warmup operation.
If fuel delivery was an issue, barely enough fuel to idle, it would not have enough fuel to get up the driveway. I'm not saying "don't test for that" but I'd look a few other places first.

walfreyydo

Measuring fuel pressure isnt really that hard and is essential to determining poor running issues - mainly the condition of the FPR, whether the pump is producing the correct pressure, and whether there are any obstructions.

Even better is installing a gauge directly on the rail.
Gauge: Adapter:
Cost: approximately $20

Result (yes this is an S2 but also workable for 8V NA):


The benefit is always knowing if you have fuel pressure and what the pressure is.

Cameronjames

Old strainers can break apart and cause pump to fail.