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RPM dips when coming to a stop quickly (850)

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LittleEndian
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RPM dips when coming to a stop quickly (850)

Post by LittleEndian » Thu Sep 13, 2018 4:28 pm

Hello,
I'm experiencing following issue, when driving and starting everything is fine, but if I slam on the breaks for quick stop, once car comes to a stop, then RPM drops and vehicle behaves as if it's about to stall, then RPM climbs back up and its OK. If breaks applied slowly, then issue does not manifest. So in short:
Quick stop = low RPM, rough, then OK.
Slow stop = no issues.

Recent work:
- Replaced PCV system
- Changed oil
- Replaced thermostat and temp sensor (coolant is reused)

What I did to try and fix the issue:
- Cleaned throttle body (spayed and wiped all the gunk and restored to clean condition, as much as possible)
- Cleaned MAF (spayed with MAF cleaner, let it dry and spayed again)
- Checked air filter, it's clean
- Yanked idle control valve from another car and installed it, same behavior (it's used, so it might be faulty as well)
- Did break booster check, not sure if I did it right, I pressed the breaks several times when car is off, and it seems to hold, started the car while holding it, and pedal went all in, turned the car off, pressed the pedal several times and seems to hold the pressure without giving in.
- Not sure if related but the little door at the bottom air box seems to be stuck open when car is off, or potentially permanently.

There is no engine light. What else can I do to resolve the issue?

According to abscate, unplugging MAF while car is on is a myth and a bad idea to do.
Update #1: read on this forum that one way to check if MAF is good is to unplug it when car is running, and if it doesn't die immediately then MAF is bad. Did just that, and from outside it felt like RPM dipped but the engine keep on working, with what seemed like occasional RPM dips. So if engine should shut off when MAF is disconnected then this might be the issue.
Update #2: Switched MAF sensor with one I had laying around, and there's no change. If I unplug it, from initial RPM dip, engine keeps on running. So unless second MAF is bad as well, or that's how its supposed to work, then issue seems not to be with it.

Update #3: For people coming in with same issue, my problem ended up being transmission fluid that leaked out and ended up at low level. Which caused RPM dips when breaking quickly.
Last edited by LittleEndian on Mon Sep 17, 2018 1:05 pm, edited 3 times in total.

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abscate
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Re: RPM dips when coming to a stop quickly (850)

Post by abscate » Thu Sep 13, 2018 9:58 pm

Update #1: read on this forum that one way to check if MAF is good is to unplug it when car is running, and if it doesn't die immediately then MAF is bad.
Let someone find this, cross it out, and delete this myth. It is also bad practice with electronics.
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Re: RPM dips when coming to a stop quickly (850)

Post by WhatAmIDoing » Thu Sep 13, 2018 10:13 pm

Just looking at the obvious, but how much petrol is in the tank? Slam on brakes with low fuel = all petrol going to front of tank = potentially starving fuel pump. Also on this, could it be possible that a hard stop would force fuel into the evap canister and flood the engine with petrol? Check the battery cable to fuse box for cracks and corrosion and check the engine grounds. Hard stops could be agitating loose connections. Last thought, when was the ATF last changed?
'98 S70 T5M - 265,000+mi

LittleEndian
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Re: RPM dips when coming to a stop quickly (850)

Post by LittleEndian » Fri Sep 14, 2018 11:46 am

WhatAmIDoing wrote:
Thu Sep 13, 2018 10:13 pm
Just looking at the obvious, but how much petrol is in the tank? Slam on brakes with low fuel = all petrol going to front of tank = potentially starving fuel pump. Also on this, could it be possible that a hard stop would force fuel into the evap canister and flood the engine with petrol? Check the battery cable to fuse box for cracks and corrosion and check the engine grounds. Hard stops could be agitating loose connections. Last thought, when was the ATF last changed?
Petrol = half a tank
Yesterday noticed grinding noise when switching between D->R and R->D. This morning tried it again, and instead of grinding noise my car jerks a little bit when shifting Park->D, Park->R, and reverse. So maybe ATF have something to do with it.

Have to wait on hurricane to pass in order to continue looking into it.

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Re: RPM dips when coming to a stop quickly (850)

Post by amblerman » Fri Sep 14, 2018 2:48 pm

abscate,

Is your comment above just about unplugging the MAF while the engine is running or is it about unplugging a MAF in general?

For example, in your experience is there any valued gained from :
1) shutting car off
2) unplugging MAF
3) starting car and driving it to see if the car's behavior changes?
4) shutting car off and connecting MAF again.

I'm asking because in another thread I mention I have symptoms but I have a 1999 so I'm trying to figure out if it's my ETB or my MAF. the ETB can't be the cause here with LittleEndian's 850.

But I already have a XemodEx ETB.. and I ran through the diagnostic flow chart but so far no obvious errors or problems.

I have read enough anecdotes about running the car without the MAF plugged in but I don't want to try it if it's a bad idea or if it will confound my issues or my debugging

-AmblerMan

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Re: RPM dips when coming to a stop quickly (850)

Post by abscate » Fri Sep 14, 2018 3:15 pm

It is usually confounding.

If you see my thread when my turbo inductions hose popped off (nice way of putting that I let the clamp rust off) when I unplugged the MAF, the car ran well - all the way home from BOS, 200 miles.

So conclusion, if car runs well with MAF unplugged, MAF is faulty, right? Here is why thats wrong.

The MAF gives the ECU the first guess at the amount of air going into the engine. From this guess, the ECU meters fuel to match the correct fuel/air ration

The OXS measures the oxygen left over and fine tunes the fuel adjustment.

If the MAF is missing or the signal is bad, the ECU guesses the amount of air based on rpm and actually gets it pretty close.

Here is the problem. The MAF can only measure air before it, so if there is any vacuum leak or induction leak, the guess it gives to the ECU is wrong, and the engine runs poorly.

When you unplug it, the ECU uses the rpm guess (in firmware in the ECU)and the engine will run well.

In either case, the MAF can be perfect, but unplugging it will fix an apparent bad running problem.
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LittleEndian
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Re: RPM dips when coming to a stop quickly (850)

Post by LittleEndian » Mon Sep 17, 2018 10:25 am

Small update, got some AT fluid:
- Drained transmission - what I got from drain did not amount to even a gallon.
- Fluid color was dark mahogany and not cherry color.
- Found what seems to be a leak on the upper radiator hose (underneath air intake) that leads to transmission. Ordered seal kit from FCPEuro, hopefully that will fix the leak once I replace o-ring.

Plan to leave new ATF for few days then when seal kit comes in, drain ATF and refill it again.

LittleEndian
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Re: RPM dips when coming to a stop quickly (850)

Post by LittleEndian » Mon Sep 17, 2018 1:01 pm

So after driving for a bit, it seems the culprit was the transmission fluid that leaked over time. After putting around 3 liters in, quick breaking no longer causes RPM dips.

But one thing still seems off. When switching between P->R, P->D and reverse, there's jerk, and when putting to park there's a thump sound. Sometimes there's grinding noise when shifting to reverse or drive. Could it be there's still not enough fluid? Car drives fine, and I don't feel anything wrong while driving. Checked transmission torque mount and it seems to still have "solidity" to it, it has a little play, but I think that's normal.

I tried to check the level and it seems all over the place. I parked on level surface, shifted between P->R->N->D->E->L and back with 5 sec delays in-between. Waited few minutes and while car was idling went to check the level, after clearing the stick and putting it back in, stick came out clean, put it back in, it came out with a little bit of fluid on it. After coming home, I did the same thing, and it seems to work fine with fluid being at HOT level, but then I noticed that my drive way is at the angle, so that measurement seems to be off.

How long do I have to wait between putting in the dip stick and taking it out? It seems if I put it in, wait 5-10 seconds, then measurement is different compared if I put it in and take it out.

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Re: RPM dips when coming to a stop quickly (850)

Post by BlackBrick » Mon Sep 17, 2018 2:59 pm

if you've been running on that much low fluid for a long time, you could have done transmission damage.

Also, thump sounds for the A/T could just be engine mounts or the transmission mount. Check the top one, the trans mount, and the front one (passenger wheel well). The trans mount is notorious for going soft and causing jerky shifts.

I run Valvoline MAXLIFE ATF Synthetic in my ATF, and it shifts perfect.

LittleEndian
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Re: RPM dips when coming to a stop quickly (850)

Post by LittleEndian » Mon Sep 17, 2018 5:26 pm

BlackBrick wrote:
Mon Sep 17, 2018 2:59 pm
if you've been running on that much low fluid for a long time, you could have done transmission damage.

Also, thump sounds for the A/T could just be engine mounts or the transmission mount. Check the top one, the trans mount, and the front one (passenger wheel well). The trans mount is notorious for going soft and causing jerky shifts.

I run Valvoline MAXLIFE ATF Synthetic in my ATF, and it shifts perfect.
I looked at passenger wheel side mount and it seems to be fine, front engine mount seems to be in whole condition, not sure about the back mount since its hard to get to. As for trans torque mount, I took it off earlier and the small rod bushing had some play to it, from what I read online, they are not supposed to be stiff as a rock and some play is expected. Is that information wrong and new trans torque mount supposed to be stiff?

PS: Not sure how long the car been driven for on low transmission, car been sitting around for more than a year after owner passed away, and I'm trying to get to running condition.

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