850 Turbo Slightly Rough Idle

[ZionXIX]

I have decided to open a thread as this rough idle problem is getting a little out of hand. Normally I am a proponent for diagnostics over parts. At least that is how this started.

Scenario: 1996 850 Turbo Wagon with ~212K miles. Car sat for nearly a year before this project.
Recently replaced all front suspension components including the following:
control arms, sway bars, front and rear struts, spring seats, (the whole nine yards), CV axles, brake pads, rotors,
Dropped the subframe and replaced the motor mounts and the power steering rack.

After the work the engine had a rough idle that smoothed out when any gas was applied. She drives great. No hesitation, no misfires while driving around. At idle after a few minutes sometimes the engine will bog down and nearly stall but will catch itself and return to idle speed. Occasionally holding RPM at ~2500-3000rpm at idle will through a misfire code on cylinder #2. But idles jumping between 800 and 840 rpm.

The suspects include the following: Idle air controller valve, dirty throttle body, MAF, cam position sensor, throttle position sensor, vacuum leaks, ECT sensor, oxygen sensors, fuel injectors, fuel pressure regulator, spark plugs, spark plug wires and more.

I have read through a few pages of the repair manual and performed the following diagnostics over the past month:

Compression test: 150psi cylinder 1-4, 135 psi cylinder 5

Multiple smoke tests: Replaced the entire intercooler hose run due to multiple leaks with Do88 hoses. Repaired leaks in the take pipe by replacing the PTC valve rubber grommet and applying gasket maker to the TCV vacuum attachment. It now passes smoke testing.

MAF sensor states in the manual to backprobe for voltage between 0.1 and 0.2 volts. Results: 0.16 volts. Cleaned with MAF cleaner. The sensor reads 5-6g/s when idling which Ive been told is too high but he manual calls for 2-7g/s.

Idle Air Control valve states in the manual to have resistance of 9-13 ohms. Results: 11 ohms and valve operates freely. Cleaned and reinstalled.

ECT: ohms match repair manual numbers based on temperature. Also replaced ~ 6yrs ago with blue box volvo. Dash gauge is working as expected and temperature number in OBDII reader also look reasonable.

Throttle body: Cleaned in place with brake cleaner and shop towel

Fuel pump: Pressure holding steady at 40 psi while running, no changes during rough idling.

Fuel pressure regulator: No fuel behind vacuum line connection. When return hose is pinched, fuel pressure rises instantly to 80 psi. So fuel pump is strong.

Spark plugs: Volvo brand installed ~6 years ago. They appeared dirty but in good condition. no evidence of cracked porcelain or fuel fouling. Replaced anyway with bosch equivalent.

Spark plug wires: Replaced with bougicord ~6 years ago. Removed and inspected for cracks, break or evidence of arcing. All clean. Tested resistance of each wire ranging from 2700 ohms for the shortest wire and 3500 ohm for the longest. No changes to resistance when flexing and pinching wires. Re-installed.

Cap and rotor: Replaced ~6 yrs ago with bosch. Removed and inspected. Mild carbon buildup on contacts. Cleaned with 220 grit sandpaper and reinstalled.

All engine grounds have been cleaned.

Fuel injectors: Removed injector wires one at a time while engine running. Engine RPM changes on all cylinders. Injectors are 30 years old original. I was planning on replacing anyway since new injectors are nearly the price of cleaning. So replaced all 5 injectors with new from rockauto.

Camshaft position sensor: Appeared clean inside with only a tiny bit of dry oil residue at the bottom of cam opening. The connector had already dissolved long ago with just wire stuck into the plug. Replaced with Bosch from rockauto.

Throttle position sensor: Replaced from rockauto just in case

Oxygen sensors: Replaced with bosch brand from rockauto.

Yes I fired the parts canon but I was intending on refreshing a lot of these parts regardless. With that said none of these fixes or replacements have changed the rough idle.

[ZionXIX]

With the impending winter storm, I'm taking some time to continue troubleshooting the rough idle. I was reading about excessive fuel vapors maybe making their way to the intake manifold could alter the air/fuel ratio. I tried disconnecting the evap purge valve hose from the intake manifold and held my finger over the vacuum leak I had just created. No difference in the end.

Then I decided to unplug the ECT and check the resistance again. Resistance is correct so the ECT must be good still. Now my ECT connector crumbles years ago and the wire barrel ends have just been direct connected the ECT for some time. The end of 1 wire finally broke so now I looking for a good universal 2 wire connector pigtail to install.

Might try cleaning those hard to reach ECU grounds near the starter.

[jreed]

I'm impressed by the thorough diagnostics and extensive checks you've done. I would have figured that any one of the items above would have been the cause of the rough idle. I know on my car ('97 855 GLT, ~260k miles) that the original idle air control valve, though spotlessly clean and moving freely when out of the car, would sometimes stick and cause rough idle and stalling during warm weather. Replacing the valve with another used one fixed the problem.

[ZionXIX]

I'm impressed by the thorough diagnostics and extensive checks you've done. I would have figured that any one of the items above would have been the cause of the rough idle. I know on my car ('97 855 GLT, ~260k miles) that the original idle air control valve, though spotlessly clean and moving freely when out of the car, would sometimes stick and cause rough idle and stalling during warm weather. Replacing the valve with another used one fixed the problem.

I'm considering replacing it regardless. Just for fun I checked the crank position sensor and it shows 312 ohms which is normal per the manual. I also decided to double down and clean all the hard to reach ground points on the lower block behind the starter. There are 3. Pulled the intake manifold for better access. Also cleaned the starter cables, alternator connections and knock sensors. I swear every connector I open on the car is filled with green corrosion. Rigged the ECT with some Wagos I had lying around but will wire in a generic pigtail after the storm lets up. I don't suppose fuel vapors could be getting through the purge valve and into the intake manifold causing a rich condition?

Oddly I tried running the engine and then used a hose pinch clamp to partially restrict the idle air control valve air supply. I swear for a few seconds it was running much better. Will definitely need to source a IAC to test theory.

[jreed]

I agree about the green corrosion. Most or all of the connectors on my car have it when I open them up, even inside areas that have a seal. I clean it out and then spray some CRC 2-26 or LiquiMoly Electronic Spray.
I like the spring support on the PCV tube -- that's a great idea.

[ZionXIX]

I agree about the green corrosion. Most or all of the connectors on my car have it when I open them up, even inside areas that have a seal. I clean it out and then spray some CRC 2-26 or LiquiMoly Electronic Spray.
I like the spring support on the PCV tube -- that's a great idea.

Thats also a larger 5/8 hose with a spring inside to prevent collapsing.

[ZionXIX]

Following a theory that maybe im getting unmetered air through the evap system. I decided to investigate the evap purge solenoid and the evap system as a whole. The purge solenoid is working. Its closed when off and opens if 12v are applied. There appears to be a 1 way valve between the solenoid abd the intake manifold, although Im able to blow a small amount of air through the valve in the wrong direction. I smoke tested the evap system before the purge solenoid and found a large crack in the plastic hose leading to charcoal canister. When trying to handle the hose, it broke easily. So now im in the market for some new hoses. Has anyone has luck using silicone vacuum lines for the evap system?

I dont know when the purge solenoid opens but when it does, perhaps some air is getting sucked into the intake.

[jreed]

I can recommend thick wall silicone hose for the evap system rubber hoses and elbows -- I replaced all of the elbows and rubber lines more than 10 years ago and they are working fine with no failures noted. I haven't tried using it to replace the hard black plastic line but I think it would work. You may want to put braided nylon sheathing over the silicone to protect it from abrasion.

[abscate]

Silicone will work fine for the evap system. Your purge valve stays closed at idle except for evap system testing. I think you have found your idle problem

[ZionXIX]

The rough idle saga continues.

Replacing the evap hose did not affect the idle. I did verify the purge valve is working when voltage is applied but by design stays closed at idle and it is closed at idle so its not really playing a role in the current problem. But my garage does smell a bit less like fuel vapors now.

I ended up using some generic thick walled vacuum hose from oreillys. 1/4 inch was perfect.

I also removed the intake manifold and cleaned all the hard to reach ground cables. There are 3 attached directly to the block, 2 behind the starter and 1 below the PCV box near the oil pan. Cleaned the alternator wire and starter wires while I was in there. And of course if you're this deep in, you might as well remove and clean the PCV box. Mine only had some minor gunk in it. A little brake cleaner took care of that.

Also since my ECT harness was broken, the wires are very easy to access with the intake manifold off. I could replace the ECT as well without removing the coolant hose but chose to leave it since its working fine. If I found my self in a situation where the thermostat housing bolt were seized and you needed to change the ECT, removing the manifold really isn't that bad. The gasket is cheap and the fuel lines are very flexible once the clamp is removed. I just left my gasket in place since it was undamaged.

For the ECT wiring repair, I used a generic 2 pin pig tail harness from Oreillys. The wire gauge is overkill at 14 gauge but it has given me any problems. And this is probably a bit safer than the Wago nuts I had placed temporarily.

Got everything re-assembled and performed a test idle. No changes to idle after ground cleaning and ECT wiring repair.

Next I performed another smoke test. The pin hole oil drain in the intercooler always throws me off because smoke will come shooting out of there. I have to put duck tape over it for testing purposes.

During this test I kept getting a leak around the turbo where the intake pipe connects. It doesnt matter how tight I make the clamp, if its rotated just right, it will leak. I assume after decades, the plastic is warped. I could solve this by purchasing the do88 hose ($300) or even the Snabb kit ($200) with their silicone connections. I went a slightly different path. I thought if I have to replace the intake pipe, I have nothing to lose in trying to seal the connection. I thought about layered hi temp foil tape, thick rubber bands, electrical tape, and heat shrink tubing.

First I purchased a $10 section of 60mm silicone hose from amazon and attempted to cut a small piece to fit around the turbo intlet. The silicone hose section is way too thick to allow the intake pipe to fit. I tried fillet-ing the material to get a thinner silicone ring. This was not successful as its very difficult to aim between the layers with reinforcement threads. I abandoned that idea and start looking for a "rubber-band" esque option. Amazon had O-rings that would probably work if you placed several stacked together but that option was not confidence inspiring.

Instead I thought "why not 3d printing exactly what I needed out of TPU?". I designed a simple ring (hollow cylinder) with 60mm ID x 15mm in height, 2 walls at 0.4mm for a total thickness of 0.8mm and 61.6mm OD. I only needed a thin layer to help seal the intake pipe.

Not only did this work, but it held enough positive pressure to blow a glove off the intake pipe. I don't know how long it will last but its potentially saving me $$$.

Total cost was 2.8grams of material with cost of $0.07 USD. Its really difficult to photograph that turbo inlet but in the 2nd picture you can see the TPU seal in place with about 5-8mm of uncovered flange. The intake pipe only make contact closest to the turbo anyway and does not use the the entire flange depth This worked perfect but If I make another one I will increase it to 20mm.

Lastly I did end up purchasing a new idle air controller just to see if that fixed the idle. It did not.

In frustration I decided to just go for long a drive last night and enjoy the ride. The car runs spectacularly. Boost pressure seems a little low but still performs great on the highway. Just idles like crap. Im only using the stick boost gauge and I know its not a good reference but I cant seem to get beyond 1/3 of the white area even at WOT. Im not sure if there if the waste gate needs adjusting or if my TCV is bad. Just more things to tinker with I suppose.

My next diagnostic is likley going to be headgasket testing. My new theory is maybe a tiny gasket leak is allowing just enough coolant into the 1 or more combustion chambers to alter the idle ever so slightly.