89 240 F+T Issues. Bogging, Backfire. Missing when cold.

I?ve been searching and reading for a good while trying to resolve the issues on my recent +T. Just want to document some of it in a thread and try to get some fresh ideas.

Car is an 89 240 LH2.4 with turbo goodies off of a 940. 13c turbo with 90+ manifold, 3 bar fpr, and 850T orange tops. Pretty solid stage 0 with BRP7ES gapped to .028, new wires, cap and rotor. No cat, unknown o2, runs worse with MAF unplugged. MBC at 10psi is right now steady.

If the engine is dead cold it will hesistate and shake when applying throttle by hand. Once it?s warm it will take fuel no problem. Other issue is that once it?s warm, the car bogs down with anything greater than minimal throttle going into boost. If I don?t let off the throttle, it will backfire. Other times it boosts well and runs pretty nicely. Also seems like the turbo is pretty laggy, especially considering it is a 13c. A strong noticeable transgression into boost. On stock boost I don?t seem to have the bogging issue, however the cold idle problem is still there.

Not really sure where to go with this other than checking the o2 voltage and resistance on the coil. Possibly the power stage as well. I?ve searched through a lot of threads and haven?t found much in the way of consistent fixes. Not really trying to dump much money in the form of parts throwing or a wideband. This is a super budget build and rusty at that. Any help would be greatly appreciated! Thanks TBers!!

Where is your MAF in proximity to the turbo?

At the end of the 940 rubber intake hose. I do have a plastic 45 degree elbow off the old 240 air box that is running from cone filter to MAF though. Keeps the filter out of the fender.

it's lean everything you said points to lean cold start, acceleration and popping out the intake

I thought so too, but the plugs had a light soot on them when I checked their condition. What are the main culprits for a lean condition? Vacuum leak/unmetered air? I can’t find anything. What about o2 sensor? Main/transfer pumps??

Have you noticed if its affected by how much fuel is in the tank?

My car did the same **** when i first turbod it. Lh2.4 ran like crap for like a day and then it magically started running great. No BS.

Doesn’t seem to show up any more or less with fuel level. Only once it’s been heat soaked and I’m messing with it. It’s been turboed for close to 3 months. Never started with the bogging until the boost came up a bit.

Checked the fuel pressure tonight, 40psi at idle and 50psi at WOT. O2 sensor voltage swept from 0.0 to 0.9 pretty continuously. Mainly in the .3, .6,.8 ranges, depending on throttle position. Don’t have any codes, but the car did set 1-1-3 at one point while cruising off throttle. Reset it and it hasn’t come back since.

1. Check for boost leaks.
2. Check for spark blowout (a bunch of cars on high boost run garbage until they warm up... though... not that garbage.)

Tested it with a boost leak detection kit used on diesel trucks to 20 psi and found no leaks in the system. I can gap the plugs a little tighter and see what happens...

How do I check for spark blowout?

I don't think there's really a way. If the car is warm, and the plugs are gapped correctly (lots to read on this) and the car falls on it's face (like traction control kicking in) then you're dealing with spark blowout. That all said. That's not the most scientific way to detect it... but without detcans or something... not sure how else?

You could try turning down the boost.

If anyone is interested, I simulaneously swapped a suspicious power stage along with upgrading to a white label ecm and my problems completely disappeared. Not the best diagnostic practice, but it was my best guess and I was able to pull those parts for a great price at a local pull and pay.

ScotchWhiskey said:

If anyone is interested, I simulaneously swapped a suspicious power stage along with upgrading to a white label ecm and my problems completely disappeared. Not the best diagnostic practice, but it was my best guess and I was able to pull those parts for a great price at a local pull and pay.

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Interesting, glad it got sorted, based on what I read as your symptoms I would have bet money it was a bad in tank pump or connector hose.

Did 940 turbos ever come with a pink label ecu?

ScotchWhiskey said:

If anyone is interested, I simulaneously swapped a suspicious power stage along with upgrading to a white label ecm and my problems completely disappeared. Not the best diagnostic practice, but it was my best guess and I was able to pull those parts for a great price at a local pull and pay.

Click to expand...

Weird, when my power stage went bad, it would just shut off completely at random.

PromiseRing said:

Did 940 turbos ever come with a pink label ecu?

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I don’t think so, but I’m just a novice. I replaced a green label 563 I had bought off the forums with a white 937 from a 1992 940 turbo.

ScotchWhiskey said:

I don?t think so, but I?m just a novice. I replaced a green label 563 I had bought off the forums with a white 937 from a 1992 940 turbo.

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Oh wow, i didn?t even know there was such thing as a green label. With a 937/954(?) ecu you should be fine. They?re supposed to be stupid reliable. Good find!

Let’s hppe so. Runs really great at 13psi for now. Looking to install a 15g or 16t with larger downpipe and exhaust in the near future. Trying to find the limits of the stock ecm and intercooler set up and hopefully be content there.