Pop under boost now runs rough.
 

94 fd with stock twins. I was under boost in a turn and heard a pop and then drop in power. Now it runs real rough. Boost still registers up to 10 psi and holds on the gauge but there's no noticeable power. Idles drops slowly to 500 and pulls about 10 in of vacuum. Real limited power accelerating from a stop until i past 3k. Even then it still feels weak.

Car was running perfectly and pulled a healthy 20 in of vacuum at idle at about 900 rpm.

Does the engine have long starts or sound like it might be running on only one rotor?

It takes slightly longer than normal to start. Not sure what running on one rotor feels like but i can drive at freeway speeds and it drives like its not boosted.

Check the intake/intercooler hoses. All of em, from the airbox to the intake mani. It sounds like either something popped off at the coupler, or you may have cracked or blown a hole in a hose. No biggie.

Sounds like a blown apex seal to me. Do a compression test to confirm.

May be easyer if you recorded it starting up

May be easy silly fix but
Hoses wont make it take longer to start

I have had my intercooler pipe disconected from elbow testing and adjusting
Made no diffrence on startup

But i have had my map sensor hose fall off and this seems to make the car sound like it runs on 1 rotor and shakes the car and will cut out unless you hold peddle
just some ideas hope it helped

This doesn't sound good......

Agreed, your motor likely blew an entire apex seal or multiple apex seals. You've stated your boost didn't change and you still get 10 so it's not a coupler. Check the map sensor vacuum lines though. Did you recently do any mods to the car? What are the mods? Stock engine? Sorry to hear about this

You blew it

No major change in mods (i replaced air filters). It was also a fairly warm day for this time of year. It sounds exactly like an unplugged map sensor. Theta no excessive smoke, backfire or smell of gas while running and it does smooth back out decently at high rpms.

The car ran like this for me once and i think i fixed it new plugs. I did so much stuff at the time it was hard to tell what the solution actually was.

Anyways I've got the uim off to inspect the vac hoses. The boost couplers are all new with tbolt clamps and they seemed to be in tact. When it happened the pop wasn't loud or alarming and sounded/felt exactly like a blown coupler (got plenty of experience with those on another car).

I'll post a vid of startup once everything is back together.

Yes but you're boosting the same so it's not a coupler and not the map sensor you say. Also plugs don't foul in one instant and make the car act how you describe.

Here's a 5 minute free compression test without a gauge you should do next.
Unplug fuel pump relay remove all plug wires then remove one spark plug on one Rotor and crank over. You should hear 3 even pulses when a buddy cranks the car. Now put that plug back and remove a plug from the other rotor and repeat and you'll have an answer.

+1 one the comp test. Easy as can be and free. Save yourself some time and test the rear rotor first.

I blew my front rotor and had the exact symptoms, check both of them.

Absolutely.

any update? Real Curious...it could also be various other uncommon fixes but seems to point to engine.

Its not an intercooler piping or coupler problem because it wouldnt effect idle nor would it hit 10psi. I didnt see you mention if you checked to ensure the MAP sensor vacuum line didnt pop off? If it is still attached that means the only other conclusion is a chipped seal which sounds like the issue. Sorry.

Same thing happened to me once, car started to run on one rotor after a hard turn in boost, drove rough in low RPM but will smooth out on higher RPM. Turned out the secondary injector o-ring decided to fail at that moment, swapped injector and everything back to normal.

Check your injectors for leak since you already have the UIM off, then a compression test if you can't find anything obvious under the manifold.

Took some time to look over the vac hoses with UIM removed and everything looked okay. I pieced it back together and on startup it seemed to start a little easier and hold a better/higher idle. I realized that I forgot to plug in the brake booster line on the UIM so the added air somehow let the car run a little better. With the booster line plugged back in the engine seemed to struggle a bit more. In both scenarios the vacuum was erratic and the needle bounced around 10 in. Interestingly the car seemed to smooth out a little when I removed the vacuum line from the map sensor.

I have videos of startup and reving with the brake booster on and off below.


With brake booster installed


With brake booster removed

^damn - too late cause everything is back together. Lol

That was one of the things that was done earlier this year. Car ran similar to how it is now. Initially it wouldn't start but I found out it was fouled plugs. Put in new plugs and it would start but ran rough similar to how it does now (although it backfired alot more then). I pulled the injectors and found that PO thought electrical tape would be good makeshift injector o-rings. The injectors were sent out for cleaning and installed with new o-rings. Then I had the 3K hesitation issue and I regrounded the engine bay. A couple of misrouted vac lines and solenoid harness later and I had a perfectly running sequential FD (for about two months until now).

Its blown.

I guess I would be checking all kinds of other stuff too if the car had been running this setup for a while without issues and you didn't overboost. When I've blown motors it was due to overboost and there was no question about it after looking at peak boost on pfc.

Any how why not pull the plugs and turn the motor over so you can figure it out quickly?

By the way were you low on gas during that hard turn while in boost?? I run with no less than half tank at the track because I have run out of gas due to fuel slosh even with over 1/4 tank remaining for certain. Happened to me during a launch in the 1/4 mile, car fell flat in first and then when I shifted and car stopped squatting it came back to life for rest of the run.

It sounds like you have a chipped seal/seals or corner seal failure maybe. Doesnt seem like a whole apex seal is gone from the sound of it. If youre going to rebuild this motor would be a good time to stop running it.

I only had time to put it back together and take a video. Not very motivated since it's essentially storage season. I plan to pick up a compression tester in the next day or two. Plus the kids will probably prefer if I find out I need a new motor after the holiday season.

Low gas is not an issue as I had a full tank. I believe the "pop" happened somewhere around the secondary turbo transistion. High boost setting on the car was at 14.5 psi that I've run with no problems all summer. Given the colder weather I had it turned town to 12.5/13 psi at the time.

Ok I understand the motivation to ignore it for now.

That's quite a bit of boost on the stock turbos. I dont know whats worse, running 14.5 on stock twins in the summer heat or the cool weather leaning out your A/Rs. The car likely wasn't tuned safely and the cold dense air was more than the tune accounted for

Ive never heard an actual sound when I blew an engine, I wonder how common that is?

Yeah, I know the boost was high, but those were the settings when I bought the car. The owner I bought it from knew nothing and did nothing to the car so that's the way it's been for over a decade. I figured why change it if it works and I didn't know the extent of the actual mods to car.

If it has a cracked seal (if the problem turns out to be a compression issue) and the rotor/ housing are currently fine, you might be lucky enough to upgrade $400-600 worth of seals and be up and running again.