97 NA850 troubleshooting, no codes, out of ideas

[div4scpro]

okay I replaced the large diameter vacuum line that goes to the brake booster, didnt see any cracks or holes but just in case.
took the TPS off and inspected, no oil or residue from the crankcase ventilation so I didnt spray any cleaner in it and never
have before but i have sprayed throttle body cleaner in the throttle body before to clean it. did a resistance check
throughout its travel and don't see any flat spots or dropoffs.
also visually checked the cruise control lines and elbows visually and sprayed the area, no problems
correct me if I am wrong, is one of the ports on the cruise control is supposed to be open? my cruise control functions perfectly.

[erikv11]

You are correct, one of the cruise ports is supposed to be open.

[div4scpro]

more details:
after changing the brake booster vacuum line thought it was fixed, ran fine for 100+ miles, then the stumbling/surging started
again, this problem really seems to correspond to when the overall engine compartment gets hot, not just when the engine
gets up to operating temp.

questions:
is there any other component that could start to cause symptoms when the engine compartment
gets hot?
there is a shop here in San Antonio that has a dyno and oscilloscope. would that combination be the best bet for trying to find
if there is an intermittent fault in either ignition or fuel system?
if I have a rock steady 19.5 vacuum at idle, engine hot or cold, is it likely that there could still be a vacuum leak? will try to
mount a vacuum gauge in the cockpit to evaluate under road conditions and when the symptoms occur.

[erikv11]

This doesn't answer any of your questions but: a symptom that correlates very closely with overall engine compartment temperature may well be a poor conductivity like from a worn out copper contact or a pinched or frayed wire. As temperature increases, resistance goes up, then you don't get the current you need through the circuit. A classic example is worn starter contacts - the starter will only work when cold in that case.

For the record, I am not convinced it isn't the TPS, mostly because that's what it was on my car (not the best reason, I know!)

[Harpoon]

In reading this post, I am wondering if it could be your cam position sensor. I have heard of them acting up when the get hot. Just a thought...Good luck!

[div4scpro]

ok more fuel to the fire, not literally:
replaced fuel pump relay with OEM unit, just in case
performed continuity/resistance check on the part of the wiring harness that the ECT connects to, no problems there
began to suspect catalytic converter, did vacuum check to see if vacuum remained high at higher rpm as it did at idle,
19.5 at idle, after quick drop under acceleration back to 19.5 or slightly higher at rpm's up to 4000, if no other ideas I
may remove the converter to visually inspect, although I thought this was a fairly definitive test.

questions:
is there a way to do a definitive test of the cam position sensor, thats too expensive to plug and play troubleshoot?
could a bad wire/connector to an injector do this, maybe just under a load?

[Ozark Lee]

If you have or know someone with an oscilloscope it will tell you what you need to know. Beyond that you need to rely on codes.

My daughter had an iffy crank position but I never pulled a code for it on my scanner. After it kept quiting on her I sent it to the local indy shop and they, at large cost, changed both the cam and crank sensors and the car has not quit since. The cost was large but the silence is golden.

...Lee

[div4scpro]

next bit of data:
removed the catalytic converter, honeycomb structure appears intact and clean from both ends, took pictures with inspection
camera. no rattling sounds or any loose pieces falling out, don't know if a converter can be failed in the middle without
evidence at either end. can a converter's condition be definitively determined removed from the vehicle? I have read about
drilling before and after and taking back pressure measurements and using a digital IR thermometer and comparing readings
front to back but would like to get some opinions before reinstalling.

ran engine while cat removed and vacuum was at same position, 19.5, as when installed

again, the frustrating thing is not one single code to go on and as much as I hate to plug and play new parts to troubleshoot it will still be cheaper than taking it to a shop.

[rcannon]

Did you try the Oxygen sensor? I'm having mild surging too but haven't done anything. Just seems like the control is not able to accurately control the mixture and I do have 135K on it. If the sensor is dirty then it would have trouble keeping the mixture correct. The mixture is changing- it has more power, then less-- about 4s of power, 2w of weakness, repeating, gradual transitions.

[waynej]

I have very similar symptoms but mine is farther along the timeline. It now only takes 20 to 30 seconds for the problem to start after start up. In the beginning it was just a shortage of power after running a while, now RPM is limited. I have checked most of the things you have plus I replaced the TPS. I also taped fuel pressure and vacuum gauges to the windshield and drove it that way for 2 days. Fuel pressure steady at 43, posted vacuum readings but no response if they are good or not.

Thread here: https://www.matthewsvolvosite.com/forums ... =1&t=43593.

Tomorrow, if it's not raining, I will tape my meter to the windshield and monitor injector pulse width while driving, but I can only do that on one injector at a time. You can buy noid lights to check for injector pulse but they mount right on injector, thus can't be seen while driving. Was thinking of making some that mount on wiper arm but haven't found right connectors yet.