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740 '89 740 Turbo Bogs down under full throttle - see video

daniels740

Member
Joined
May 8, 2019
Location
South Florida
As I was driving home today, about a quarter-mile from home, I decided to step on the gas once a really slow driver finally made their turn. Anyway, after doing this, the engine bogged down and returned to idle despite my foot still being on the pedal. I took it off and tried to recover from the little mishap when, to my surprise, it was as if I had no accel at all. I thought the cable to the throttle had snapped. It idled perfectly fine, even with the throttle wide open.

Anyway, I limped home at about 10mph and only then I checked under the hood to see that nothing looked out of the ordinary. I went inside and cooled off for 5 minutes before going back out and restarting the car. Didn't do this earlier because I didn't even know if it would start again.

Anyway, the Volvo started just fine and I could accelerate again. However, I noticed that it bogs down when it reaches 3k rpm when I go full throttle. Here's a video.

<blockquote class="imgur-embed-pub" lang="en" data-id="a/arbhMef" data-context="false" ><a href="//imgur.com/a/arbhMef"></a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>
In this video, at the start, I punch on the gas down to full throttle. I do not let off the gas pedal whatsoever as it goes down to 1.25k. I repeat this twice. On the third time, I more gently accelerate and it goes up just fine.

Thinking this might be the overboost switch over the driver's side footwell? What else could it be?

Thanks in advance.
 
Unplugged the overboost switch. Made no difference. Also unplugged the throttle position sensor which did not change anything. On the other hand, unplugging the MAF sensor did not make a change on the first try but did on the second and third. Right now, it looks like none of those parts are to blame, at least to me.

Driving gingerly, I drove about 10 miles today with no problem at all. Would like to get this fixed ASAP, though. I cannot risk this happening on the highway or when making a more 'risky' left turn.
 
Here's a more simple question: What can make the throttle completely useless?

Even manually operating the throttle in the engine bay did absolutely nothing yesterday during that incident. Like, moving it to full throttle did not increase or even decrease rpms at all. It was as if the throttle lever was no longer connected to the butterfly valve.
 
Is the pre-turbo hose sucking shut/collapsing on itself? Any of the intake/boost hoses?
 
One time I had a 945T that had foam on the inside of the air box and it was sucking onto the MAF and choking the engine out. very similar symptom.
either way it sounds like air is not getting to the throttle.
 
It will get to 5k+ RPM if I go slowly rather than all at once, telling me it can get the air. That being said, I will check the filter box and MAF as well as spray the MAF clean with special cleaner spray like I do every so often.

Really has me stumped.
 
I haven't tested fuel pressure. However, I have a new main pump + filter (2 years old, low miles). Like seen in the video, it will go to 5k RPM + if I go just a bit more gingerly on the throttle. Goes into boost just fine, too.
 
I'd guess on a collapsing hose, but checking the turbo can't hurt -- when the engine is cold, see if you can spin the turbo impeller by hand. Also try pushing it side-to-side and top-to-bottom while turning to see if it ever binds. There will be some slop, but it should never hit the housing. Does your CEL/MIL light work, and are then any diag codes posted?
 
LH 2.2 doesn't use the CEL, right? I've never seen it. Also, I don't see any LED light indicator near the firewall to check for codes.

Wouldn't a collapsing hose cause a momentary spike in vacuum before the RPMs drop? Today, if I have the time and if the weather permits, I will check my filter, clean my MAF, and inspect all the hoses.
 
You're right, LH2.2 doesn't have a CEL. I wasn't sure when the 740s went to LH2.4.

Let's say that you're running normally at 3K RPM under boost. There will be a slight vacuum between the intake air filter and the turbo compressor intake -- the compressor is sucking in air to compress and needs to overcome the slight restriction from the air filter. There will be boost pressure all the way from the compressor outlet to the intake valves, minus some intercooler and piping losses.

Now, collapse an intake hose or throw something over the air filter passage, and all airflow gets choked off. The compressor will try it's best to keep air flowing, but is choked off, and pressure drops rapidly. On top of this, the engine is still at 3K rpm and will try to suck in more air as it spins down, which can create the vacuum spike. Once engine speed falls, the turbo may not be spinning fast enough to build any pressure, but the normal NA engine vacuum can be enough to keep the collapsed hose choked off until you get back to low flow at idle.
 
Thanks for insisting! I took another look at the hoses when accelerating and the hose going into the throttle body was sucking itself shut. Glad it's something easy!

The other day, what must've happened is that soft hose sucked itself shut and remained that way, and I didn't notice it because it was dark.

Order for the hose going in ASAP.

Thanks a bunch to all who helped.
 
Just for fun, here's the video of the collapsing hose. Took a video before R&R and thought it'd be something cool to share.
<blockquote class="imgur-embed-pub" lang="en" data-id="a/WNhvoG6" data-context="false" ><a href="//imgur.com/a/WNhvoG6"></a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>
 
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