1990 740 turbo surging.

[noz-e8tr]

Edit-problem solved. Root cause, my stupidity. When I did my freeze plugs, I removed all connections and moved the harness out of the way. In my haste, when I plugged everything back in, I swapped the CSI and knock sensor connection. I haven’t been able to pull codes because the led light wasn’t working. Fixed that, got a code 143. Then someone pointed out that the connections were color coded. Mine were so dirty I thought they were all black. Cleaned them off, moved blue to CSI and black to knock sensor and all is great. Pulling hard right up to 15-16psi.


Hey guys. My 1990 740 turbo was running strong a year ago. Recently it needed some tlc. Anytime I would go into boost it would just fall on its face. Here is what I’ve done. New plugs, wires, rotor and distributor cap. New coil. I’ve cleaned the MAF and made sure the TPS is properly adjusted. Has fresh cleaned and balanced injectors. It’s running much better, but still not right. Here is my setup.

Chipped
Ipd turbo cam
Npr intercooler
Boost controller.

What I’m seeing now is it boost smooth with 1/4-1/2 throttle. At 3/4 throttle it’s like a dead spot. Surging. Like maybe it’s too rich. Boost level is not surging, but the car is. It smooths back out at WOT but still seen a bit down on power. My gut feeling is it’s dropping too much fuel. Guess it’s time for a wideband so I know what’s going on. . It’s also started backfiring sometimes in between shifts. (M46). Anybody want to bench race this? Would a bad/failing MAF course this?

 

[dl242gt]

How are the air intake hoses? Especially the one after the air mass sensor and goes down to the turbo. It may be the classic case of that hose being sucked shut. The other hoses can make funky issues when they are soaked with oil from the inside. But that turbo intake hose is a common problem.

 

[noz-e8tr]

I?ll check that tomorrow. Thanks.

How are the air intake hoses? Especially the one after the air mass sensor and goes down to the turbo. It may be the classic case of that hose being sucked shut. The other hoses can make funky issues when they are soaked with oil from the inside. But that turbo intake hose is a common problem.

 

[gsellstr]

What boost level does it start acting up? Stock turbo?

That's very familiar to what mine felt like on knock enrichment. Just kinda flat but it was fine if you backed out of it.

Also I'd check the inlet hose to the turbo since they get soft and can cause issues, like Dave mentioned.

 

[noz-e8tr]

Seems to act up around 4-6psi. Stock turbo. It was boosting smooth to 15psi no problem. It progressively got worse over time until I did a stage zero on it. Things improved after the stage zero, but still have this flat spot where it feels power is being cut. I’ll check the inlet hose tomorrow. What was the source of knock enrichment? Did you actually have knock? Or was it a bad knock sensor?

What boost level does it start acting up? Stock turbo?

That's very familiar to what mine felt like on knock enrichment. Just kinda flat but it was fine if you backed out of it.

Also I'd check the inlet hose to the turbo since they get soft and can cause issues, like Dave mentioned.

 

[ZVOLV]

Air leaks. Smoke test. Or homebrew pressure test. Visual test places such as IAC hoses. Mirror and flashlight around areas.

 

[soclosenotnear]

Definitely had that issue with an intake hose before. Acted pretty much as you describe. If it's of unknown age and/or soft, I would replace it.

 

[gsellstr]

Knock enrichment comes into play when the EZK senses knock, pulls as much timing as it's allowed, then it signals the LH box to dump fuel since it's still sensing it. Never happened below about 10-12psi though, and it was related to the spark map not being quite right for that boost on our crap gas. If you're seeing it at 4-6psi, you've got something else going on.

Those symptoms do kinda sound like it's too rich.

Cap and rotor in good shape, wires in good shape, no external leaks and no high resistance?
Ignition coil resistance on primary and secondary, both cold and hot? You mentioned a new coil, stock or aftermarket? Mine absolutely hated the new Accel I put on it when in boost, no issues with the BlasterSS.
Definitely air leaks, including IAC hoses and such, brake booster, all of it.
Fuel filter of known mileage?
Fuel pressure?

 

[noz-e8tr]

All my turbo hoses are factory stock from 1990. The inlet hose going into the turbo is very soft. I just ordered a complete silicone hose kit. Figured it way past time for that even if it?s not the issue.

Definitely had that issue with an intake hose before. Acted pretty much as you describe. If it's of unknown age and/or soft, I would rept it.

 

[soclosenotnear]

All my turbo hoses are factory stock from 1990. The inlet hose going into the turbo is very soft. I just ordered a complete silicone hose kit. Figured it way past time for that even if it?s not the issue.

I once bought a car that drove fine during the test drive. As soon as I started my trek home, with a hot car at this point, that hose started collapsing. I was able to make it hold its shape with a hose clamp well enough to get me home. But you won't be disappointed replacing them. I'm betting that's the issue.

 

[gsellstr]

Entirely possible that's it, but either way that specifically is good insurance. Most common failure is the bottom side where the oil sits and rots away at it, combined with the heat. One of the parts cars I grabbed years back was a 'non-running' car. Got there, guy asked where the trailer was, told him we wanted to look at it first. Borrowed a battery, started it, idled but wouldn't rev. My friend went over to ink the deal on the car while I went for a screwdriver. Loosened the clamp at the turbo and AMM, pulled it, trimmed the mush off a little, jammed it back in, ran perfect, so the guy tried upping the previously agreed upon price. Nope, sorry dude, handshake deal was shaken on already, so we drove it home.