My 1989 740 Turbo

I'm really at a loss with this one. My car misfires once or twice when I drive it. It regularly happens in the first 15min of driving. It's not while the engine is under a boost of any greatness, but sometimes it can jerk the car a lot. It's only been doing this since this winter. It's not know since I don't hear anything like a pinging. I don't really know what one sounds like, but it never hear anything unusual. I recently got the IPD wires and new Bosch plugs. I gaped them properly. It's exactly the same. The cap a rotor look fine. A little worn, but not as bad as the original I replaced a few years back (which had no problems when I changed it). The old plugs were almost double the specified gap too. I put in a new oxygen sensor since the gas milage is down a little and I thought it might help my hunting idle (223k on the original). No difference. This little gremlin just popped up and I still haven't fixed the hunting idle that's plagued this car for years. And It's still a fast solid and reliable car.

So any comments are welcome!

 

When the motor misfires does it sound like the whole engine's breaking up or just more of a specific or random miss? Does it go away when you let off the gas? Does it stop when the motor warms up? As for hunting idle, check out the cure I used for almost a year...

Hose clamp pliers...see the handles sticking up behind the p/s pump? I got pretty good at adjusting them depending on the weather!!

 

I personally would get rid of the Bosch plugs.... for NGK

 

Nothing wrong with Bosch coppers..although the ++ pretty much blow. I've had good service from Bosch, NGK and Nipondenso. Detonation can fubar any plug though. Now, plugs that should never be seen in a red block are Champion, AC or Autolite. Don't know what it is, but Champions in particular do not do well at all in Volvos. That's not to say you might have a bunk plug...crack in the ceramic is common result of detonation and no one makes plugs as well as they used to...impossible to see often too. When I have little mysteries like this I'll wait until it's dark outside and pop the hood while motor is running...real easy to see leaking ignition wires!

 

I've heard that some people are partial to NGK. I like them. I run them in all Hondas I've ever touched. The old plugs in the car were NGKs. I did clean them up and re-gap them, but the problem persisted. The Bosch plugs are WR 7 DC, so they are the right type. The ignition wires are new so they shouldn't leak, or at least I would think. The symptoms haven't changed a bit, so I think it may be elsewhere, but you got any ideas to see if a plug has a small crack?

The miss is very random and I don't hear anything slightly suspicious of detonation. It's just a big jerk. having a manual probably makes it feel worse than an auto, but still, it a healthy jerk sometimes. It happens only when I am giving it gas, but not very much. I've never had it happen at WOT and I think most of the time the manifold pressure is below 1atm. Then I can mash the pedal to the floor and there will be no problems. It does not seem like knocking to me.

Could water in the fuel cause a misfire like this?

It does miss when the engine is warm, but regularly when warming up. It seems to happen only at the beginning of trips. It would seem like it has something to do with engine heat, but it has happened a few times when the engine is reading full running temp, but after being warm for some time it never seems to miss.

I like the idle solution. I thought about getting a valve to put in place of the IAC to accomplish the same thing. I'll tell you a little more about what I did with that. My friend used to have a 98 760 turbo (which he traded on a Toyota for 100 bucks!!! It was supposed to become mine, I would have paid more) so I swapped the IAC. The problem stayed in my car, and did not go to his car. I did not "reset" the idle, because I did not know how to do that back then. Maybe I should have, maybe it won't make a difference. I'm leaning toward it won't make a difference, but maybe one of you would know. When I ground the IAC test point to close the valve it still hunts. Then when I disconnect the ground, the engine races for a second and then returns to normal.... hunting. The IAC seems to be okay. I'll give the clamp things a shot anyway, I have a pair.

A few other notes about the hunting: It does not hunt on startup, It will idle for a few minutes before it starts hunting. When I come to a stoplight and put in the clutch, engine speed will dip below idle a little (if lights are on, they will dim a bit and the heater motor will slow for a second that seems normal for under speeding the engine), then it will idle normally for maybe 10 seconds and then destabilize and hunt for the right idle speed again. It will start small and then in about 5 seconds it will be at full hunting.

So there's some details for you to chew on. I may buy another set of NGK plug, as they are cheap enough. Thanks thus far.

 

Hmmm...a bad idle circuit in the ecu is possible too. 541's are still plentiful and cheap. The 941? was the dealer replacement for warrantied 541's...a bit more stable and easily chipped. There's a 541 on eBay right now for $35 shipped and warrantied! http://cgi.ebay.com/ebaymotors/VOLVO...Q5fAccessories
When I sold the 760 last fall the pliers went with it! The IAC was new, ecu was new, hoses and TPS new...but it was an 84 and had the horrible wire insulation. I'm gonna have to ask my brother-in-law about your misfire situation.

 

This got me thinking: I have an '87 manual turbo that does something similar, something I attributed to tranny (overdrive) symptom rather than an engine miss... It is a brief momentary "skip" which could very well be something like we are talking about here. Mine only happens at freeway speeds after the car is warmed up, 15-20 minutes from start up, and when 5th is engaged. It's been on my list to check the trans fluid but it could be just an engine miss as well...

Correct diagnosis is tricky, but it is more than half the battle.

 

When I was swapping out parts with my friend's car a few years ago I swapped the ECU and MAF separately. Neither made a difference with the idle. Should I not eliminate that? I'm still thinking that there could be a well hidden vacuum leak. But that won't help the missing.

I have not checked valve lash. I did a compression check years ago when the hunting idle started and here's the numbers I got 1=149, 2=150, 3=155, 4=166 (psi).

I've thought about the wire harness, since it is one of the few things that is really hard to check. I also know that this year is not prone to wire harness failure. If I can't fin the problem anywhere else, I was going to rip it apart, inspect it and find new conduit to put it in. That should not be that hard if I can find good conduit. But I'm not at that point yet.

The misfiring is happening in lower gears, since I'm doing a lot of city driving. By the time I get on the high way and up to speed, it doesn't seem to happen. My car was having some problem about 10 min into a drive with dropping out a 5th gear (several years ago). It turned out to be chaffing wires in the shift lever.

That about sums it up.

 

You hit the two items I had in mind: valve lash & vacuum leak.
Valves get tighter over time and as they get warmer.
Ed

 

At this point I would consider checking compression again...

 

So this is on my to do list now:

Compression Check
Valve Lash Check
Another Compression Check
Another vacuum check from the intercooler up to the MAF

I was just looking around on Dave Barton's website and saw that he now sells conduit for wire harnesses. Check it out: http://www.davebarton.com/blackvinyl.html

I'm hoping it won't come to that.

That, I was never aware of. I know that with heat they expand, and with time they may need adjusting, but that is new to me. Good to know.

 

So here's the results of my compression test:
Old test from about five years ago are to the far right, the one I did tonight is on the left.

1=125----144
2=128----150
3=135----155
4=138----166

The old test I think I did with the engine cold and with a Snap-On tester.
The new test I did with a warm engine and a cheap Harbor Freight tester.

Well compression does seem to be down. This car has never had valve lash adjusted, so that sounds like a good place to start. Out of curiosity, what normal for compression?

 

Meh...I mean, what is normal anyway?? Seriously, the main thing to learn from a compression is not really how high the individual numbers are but rather how close they are to each other. New, compression may be as high as 180ish. My 80 GT's motor had 250,000 miles on it and tested w/i 2 psi of 140 across the board. Numbers of a somewhat tired motor but wearing out evenly. 160's are nice to see but rare w/ that kind of miles. Two things dictate compression loss; rings and valves. It doesn't matter if the engine is hot or cold. Some people will argue that the test needs to be done with the throttle wide open. These people do not understand that as a diagnostic aid to engine health...it is not how high the compression is but rather how close the individual readings are. Consequently, HF or Snap-On...makes no difference. You have some stuff going on in your engine...the difference between #1 and #4 is a bit more than one wants to see. Surely this was a "dry" compression test. By squirting a bit of motor oil into each cylinder, thus helping to seal the rings, you could then do a "wet test. This will help you to determine if it's rings (numbers will be appreciably higher) or valves (numbers should stay the same). There could be a third factor affecting your numbers as well...a failing head gasket. Combustion byproducts could be entering the cooling system. Gross example pressurize the cooling system to the extent that, with the overflow cap off and engine warm....revving the motor to 3-4k will make coolant "puke" out of the bottle. Lesser leaks may be detected by chemical tests of the coolant for carbon monoxide, any shop will have the tester.

 

When I did the test 5 years ago, I did the oil trick and I got the same answers. I've got an antifreeze tester around here somewhere, I'll give that a shot. I'll run the oil again for compression. But I think I'll start with valve lash and work my way to the head gasket. If I do find the rings to be leaky, I could hone and re-ring the engine. I'm tempted to pull the engine at that point. I've got a friend with an engine lift and stand. But I'll start with the easy stuff first. I'll keep you posted.

 

you might check your fuel pressure ... if its bouncing low randomly, the regulator may be failing. my 745t misses occasionally at an idle, I haven't bothered to pin it down yet, but the fuel pressure regulator is pretty high on my list of suspects.

 

I had a pressure test done when it first started idling funny. It checked out good. I'm not completely eliminating it.

I check the valve lash. I'm probably the first to do so on the car and all are still in spec. .012 to .016 is spec and all were .015 to .016. It's on the wide end, so they are in great shape.

I ran the compression again. This time I cranked until the needle quit rising. Then I repeated with a shot of oil.

cyl dry oil
1 131 138
2 135 145
3 145 160
4 147 158

Looks like are the rings are a little worn in all 4 pots and cylinder 1 and 2 could benefit from a good valve lapping job.

I read in my book that 128psi is minimum spec for pressure and 28psi is max difference between any two pots. I'm still within spec, but not by too much.

Next up, I'll still be looking for secret vacuum leaks.

 

I plugged another vacuum leak but I can still hear one.

Okay, but back to the misfire. During the summer it basically went away. Now it's cold and it's back and worse than before. Just a little. I put my old cap on the car. This old cap never had a problem. It misfires during warm up mostly, but now when regular running every now and then. When it misfires it's just once. I'm thinking its time for a new coil. But other considerations are a new ignition module and of course a cap and rotor. I might get an ignition module from the junk yard. Maybe a coil as well from there. Any opinions or other thoughts?

 

Bump.

I'm planning on buying a new Bosch coil. I'm still listening for opinions.

 

bosch coils are pretty cheap. but they are also rather extremely simple and reliable. I haven't re-read this whole thread, but I'd point out that the ignition module is at least as likely to be a problem or the wiring and connectors.

 

oh. my 850 turbo started experiencing a very occasional little stumble, which would sometimes also trigger a OBD-II fault code which indicated the MAF output was out of spec. I reseated the connector and it didn't come back for months, then started again. then, blam, it started bucking like a bronco at speed. and yup, the MAF was fried.