Turbo Wagon bogs down/hesitates randomly when warm

Holiday · Post by **Holiday** » 22 Sep 2021, 10:40

96 Volvo 850 Turbo Wagon.

Some history:

Had a general lack of performance with the car and the odd misfire. Set an appointment for a full tune up as recommended. At some point a mechanic convinced me the catalytic converter was clogged and so I had the car straight piped till I could figure how it got clogged (later appeared the CAT wasn't clogged up after all, car is still straight piped at the moment). Shortly before the tune up I had a stalling issue when starting. Changed the fuel relay and that seemed to resolve the stalling. Then a few days later the car would start and idle really rough then die. I changed the MAF sensor (previous owner apparently put in an aftermarket one) to a Bosch one from a working Volvo similar to mine. Car then idled and ran smooth, best yet. Then I had the tune up done. The car ran amazingly zipping around town. Then one day I was on the highway for about 30 min and I noticed a sudden loss of power. It would bog down momentarily then resume normally. Sometimes it would chug 4 or 5 times then resume. Seemed to happen only when car was at temperature for extended period, driving over 100k an hour then letting off the accelerator then back on. For the first 30 min of driving it's fine then after there is intermittent hesitation.

Car was checked for vacuum leaks and it was good (very minor leak in the turbo area), PCV was also tested and it was good. The car did have both a MAF code and a VSS (speed sensor) code. Seems the clamp on the MAF sensor housing was loose so it was tightened up. Codes were clear, car ran great initially, codes didn't return. Then once again after 30 minutes or so of highway driving the hesitancy returned.

Any ideas as to the cause of the car bogging down once warm or hot?

Post by **scot850** » 22 Sep 2021, 11:48

First questions we usually ask so we have a baseline to start at:

1) What work was done as part of the tune up?

2) What brand of parts was used for parts replaced?

These cars can be very particular about what parts are used. If we can establish that the new parts are unlikely to be the issue (and considering you had an issue prior to install as I read it).

Then we can start looking at the other possibilities.

I have noted that the issue starts when the car has warmed up so that can point in a few directions.

The standard things needed for a car to run are the obvious : Fuel, Air and spark

All these issues can be affected by the heat of the engine, especially a turbo.

Neil.

Post by **BlackBart** » 22 Sep 2021, 12:46

Just a question-
Were the O2 sensors left in place when the cat got removed? Are they OK?

Holiday · Post by **Holiday** » 22 Sep 2021, 13:14

Thanks for the response. The tune up was all Bosch parts except for the coil which was some generic brand i believe. So spark plugs, ignition wiring, rotor, cap, coil done. MAF was also changed just before tune up. I don't have a dash light on but when Volvo specialist ran a check there were codes related to the cat delete. Apparently the car only has one sensor in the exhaust area, it's still there.

Codes pre-wipe were: 311, 121, 443, 435, 212.

Did another run today. Runs perfectly but as temp gets closer to the middle mark it shows subtle hesitancy. 5 minutes after reaching stable middle temps it begins to bog down and remains so intermittently. Mostly noticable at 90+ kph, the car seemed to normalize keeping at 80kph and below, in town at low speeds it seemed quite normal with no hesitancy and this is while the car is still at operating temps. I ran a code read using my Bluetooth OBD2 reader and app. Keeps saying the ECU is storing no codes. Im gonna try and get a better OBD2 reader to see if that picks anything up.

Don't know if this also helps but post CAT delete the car ran only marginally better and if you hit the accelerator quick it would make a stuttering "put-put-put" sound in the higher RPM range. When I changed the MAF that went away and it was the mostable positive change to the car's performance thus far. Then the tune up was done. Again car runs perfect initially and going around town, issues at prolonged highway speeds.

Post by **erikv11** » 23 Sep 2021, 08:33

Sounds like something that gets in trouble when both conditions are met (1) the engine/part has heated up and (2) the engine is under load or high rpm. So check out critical electrical parts affected as the heat causes electrical resistance to creep up higher; weak(ish) spark would fit the symptoms so far. If it was a sensor (MAF, ECT) then once (1) was met it would just keep acting up. The engine bay is actually a little cooler on the highway due to increased air flow but this time of year it would still stay hot in there once there is heat soak.

Bosch on the wires is still not the greatest, IMO only Bougicord is certain, but it looks like many people replace with Bosch these days so they are likely fine. Do you still have the old coil? Very unlikely it was bad. For troubleshooting I'd swap the coil back in and do the same thorough testing. Or grab one at the junkyard.

Holiday · Post by **Holiday** » 23 Sep 2021, 10:10

I do have the old OEM coil so I'll possibly have that put back in tomorrow. Would what I'm experiencing be due to the coil that was installed? Brand new but again it's aftermarket.

Post by **erikv11** » 23 Sep 2021, 13:55

Holiday wrote: ↑23 Sep 2021, 10:10 I do have the old OEM coil so I'll possibly have that put back in tomorrow. Would what I'm experiencing be due to the coil that was installed? Brand new but again it's aftermarket.

That's the idea, yes, that the new coil is not working properly under heat. New parts can be hit-miss, especially electronics, IMO they are always suspect after a job is done until they prove their mettle. I suggest the coil swap mostly because it should be free (low-cost?) troubleshooting.

Also this just occurred to me that just because of all that ignition work done, they need to carefully (properly) route the coil wire, to minimize proximity to the cam sensor and crank sensor wires on that end of the engine. The coil wire voltage can interfere with those sensor signals. Not sure how heat would play into that but it's worth checking.

Post by **scot850** » 23 Sep 2021, 15:04

I have had that issue and it was random with the coil wire and the distributor, as well as the cam sensor wire. When it played up the car could be hot or cold.

Another simple thing to try is the fuel pressure measurement (42psi in your car) taken at the fuel rail Shraeder valve. Another thing can be the fuel pump relay. There is info on how to bypass that with a jumper wire. Please not this is a diagnostic device and not a long term fix!!

Neil.

Post by **erikv11** » 23 Sep 2021, 15:42

Fuel pump relay is a good idea, thanks Neil.

Holiday · Post by **Holiday** » 24 Sep 2021, 20:02

So the car had no codes when I brought to the Volvo specialist except a new one 114 Mode Selector Switch.

I did notice something when the car was at the shop and that was the air intake tube connecting to the air box was loose and to the side of the airbox a bit not connected properly. I pointed it out the mechanic and it seemed no big deal to him and he just shoved the hose back on the airbox but didn't seem all that secure. Back on the highway the car was fine and then eventually the bogging down would return sporadically. When I got home I could hear a faint whistling sound but not sure where it was from, right side of the engine near the airbox. I looked at the airbox intake tube connection and it was loose and partially off the box.

So I'm wondering if that is the culprit. The car would be getting only a small amount of cool fresh air to the airbox and the rest would be from the hot engine. Could that be the issue?