RMills944

Sounds like maybe you just need an injector cleaning or FPR. The fuel system is my first suspect on that one since you've replaced the AFM and O2 sensor.

Thom

So Luis, have you been able to source the issue ?

Luis de Prat

ISV is in the mail from the U.S. for around €100 less than here. Thanks for checking!

Luis de Prat

I received the ISV but haven't gotten a clear response as to whether it could be the cause of the hesitation at WOT or not.

To those suggestiong FPR and damper, I know Fpena944 has been having a similar issue that wasn't fixed by replacing these.

Problem went away by itself without replacing anything on the car so I've waited to waste a perfectly good ISV until it started hesitating again: yesterday.

NOTE: When the car hesitates the rev counter keeps climbing unaltered. Sounds like if it was an electrical problem the needle would bounce, no?

Mark944na86

Luis, I know this might sound somewhat unlikely, but I thought I'd mention a hesitation problem I had on my old 86 NA.

I thought I'd never track it down. Then it turned out to be ignition system related, as in when I put in a new rotor, plugs, spark plug wire set, coil etc. it cleared up.

I did all that because I was putting in a CD ignition (MSD 6A). The MSD unit worked quite nicely, extra torque for a NA down the bottom end, but the nice bonus was the hesitation cleared up very nicely as a side-effect.

Just a thought if you've exhausted all other possibilities...

Regards,

-Mark

Luis de Prat

I guess it wouldn't hurt to try putting on a new cap and rotor, and maybe even replace the coil. The plug wires aren't going to be cheap, though.

I appreciate your help, Mark.

Roy LaZelle

Luis: I just finished a painful debugging process with one my high mileage Volvos with Bosch FI. I wish now that I had used the plug condition to narrow down the choices between rich and lean problems. My latest problem turned out to be the FPR causing overpressure (yes, too much pressure = too much gas). The car didn’t smoke but had blackest plugs I have ever seen. It also had a hesitation when flooring the gas peddle at (low speed) while going up hill. Good luck –Roy--

fpena944

I sent my fuel injectors away for a rebuild. Finally got my car back on the road today after my brake pad debacle. One of the injectors was leaking a bit, so far today the check engine light came on once (in 5 starts) and went out after about 3 seconds.

The car seems to be running okay but my heart sank after I saw that check engine light. I'm not sure what to try next but I still haven't seemed to cure the problem entirely.

Ken D

That is the 'dirty' way of testing the adjustment of the switch, though I've "heard" even that can be misleading.

There is actually a better way of testing the TPS by hooking up a multimeter/ohmmeter to look at the outputs. The full procedure is listed on Clark's site. Assuming you have a multi/ohmmeter, the test is free and worthwhile. Good luck!

jonnybgood

Yeah, Just because you hear the click does not mean the signal is making it back to the DME. Check the signal at the DME connector to be sure.

Removing the intake manifold is not as scary as you might think. Just make sure you plug the holes over the flywheel and intake ports. While you are there check the large vacuum hoses. On my car they had hardend around the connection points. They still had the original pinch clamps from the factory so they looked fine. When I removed the pinch clamps and put on new hose clamps my problems with dying on return to idle disappeared. I guess I am suggesting to check your vacuum connections if you go into the intake manifold. Your symptoms are different then mine but with 142k you should be looking into vacuum leaks too.

EDIT: If the hesitation occurs at the same RPM then I would suspect the AFM wiper was dirty or a loss of signal from the AFM.

gtroth

I wouldn't suspect the ISV for your symptoms, personally. That valve merely opens to by-pass the throttle for a little extra air when cold, and I think closes off when not idling. When warm, it opens and closes in smaller increments to adjust the idle. If I understand things, if it were sticking open, the throttle would just behave as if it's open a little wider then you commanded it to open with your foot. When it sticks closed, the idle will drop perilously close to stalling, and either it will stall or bounce to ~1500. This is I think the classic ISV failure mode.

Definitely measure the fuel pressure as in FSM's DME Troubleshooting / Dignosis section or as on Clark's. Then you'll KNOW about that.

Luis de Prat

An update.

Today I bit the bullet and replaced both the TPS and ISV on the S2. I also put in new ISV hoses, and replaced the 2 vacuum Ts.

The TPS I knew couldn't hurt to replace, but I was really hesitant to put in the new expensive ISV. I ended up replacing both, since I figured it would be too much of a hassle to send it back.

Apparantly, it worked! Driving around after the repair, the car no longer hesitates and I'm going to give it a few more days of test driving to make sure. I think the hesitation was caused by the TPS, which was original on the car at 146K (i.e. the philips screws still had the factory yellow paint on the heads).

As for the ISV, I do notice the car idles a bit lower than before and is quieter at stoplights. Nice!

I want to thank everyone who posted their suggestions and advice to this thread. Particularly Riff, for his great pics with instructions that were very helpful.