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Hardstart when cold, hesitation, stumbly idle


Active member
Feb 1, 2014
1990 740GL with lh2.4

Car sat for awhile, I put 2 new pumps and filters in it, as well as good used injectors, new bpr6es plugs, and a good ole Italian tune up. Car runs infinitely better than when i first got it. On a cold start in the mornings, it would crank and crank and crank before starting. I figured out that the fuel pump relay was lazy and not priming properly so I threw in an aftermarket one I had laying around. Sat all day today in the cold snow and when i went to fire it up, I heard the pumps prime how they?re supposed to. Turned the key and it started for a split second then died. Tried again and it started right up. So, issue number one is not wanting to start first try when cold.

Issue number 2 is a stumbly idle, very noticeable in drive. CEL is illuminated.

Issue number 3 is a hesitation. Say the car is in park/neutral and I blip the throttle heavily, it will stumble before revving up. Sounds like it?s missing while I hit the gas.

Getting a 1-1-1 code from port 6, but when I try port 2, I get 0 feedback from the LED. Not sure why.

Plans are to check closer for air leaks, clean AMM, and possibly throw on some used cap/rotor/wires. Possibly test CTS as well.

If you guys could throw me some ideas as to why it?s having trouble cold starting, hesitating, and not providing feedback from port 2, that would be great.
When pulling codes, the led very faintly illuminates in port 6 when the probe is inserted. However, in port 2 it does not. So something is going on there. I pulled box off and inspected for corrosion but found nothing.
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Does the diag LED light up when you push the button with the probe not plugged into anything? If so, it means you have good power and ground to the diag box. If not, ground is under the button, power is under the LED. You can also touch the probe to ground to light up the LED.

If the 740 harness is like the 240 harnesses, then the pin 2 diag wire runs all the way to the LH ECU with no other connectors in the path (pin 6 goes direct to EZK). Since you're getting a CEL but pin 6 shows 1-1-1, the CEL must be from ECU. Find a chart of ECU fault codes -- only some of them will light up the CEL (CTS, AMM but not TPS IIRC).
You probably have a bad connection if port 2 is not working but the ezk on port six is. Or the ecu has issues. I would also check the engine speed sensor. Even if you aren't getting any ezk codes.

You may have a sneaky vacuum leak like an intake gasket or permeated air handling hoses if that stuff is unknown service life. Don't forget a possibly loose timing belt if that is unknown service life. Maybe reset the tension on the timing belt?
I’ve already reset the tensioner when i planned on doing a timing belt before buying by the car ( it turned out to not be snapped like the previous owner had thought).

I’m betting I have a pretty leaky intake manifold gasket or something like that. I thought I checked thoroughly for vacuum leaks but it’s pretty easy to overlook some of them.

As for the diagnostics box, the LED will light up when I push the button even if the pin is in port 2, but it will not give me any feedback after that. The LED very faintly illuminates when it is inserted into port 6 but does not illuminate when in port two, so I know it has an issue. I cleaned all of the connections directly underneath the box but that did not help. Maybe I will trace that wire back to the LH computer and see if it is disconnected or something silly like that.
Have you checked compression?
Have you checked fuel pressure?

Compression is 185 across the board.

Haven’t checked fuel pressure yet but it has a brand new in tank pump and filter as well as good flowing injectors, and a ‘new’ main pump and filter. And a new relay. And a newer fuel pressure regulator. Fuel system is essentially new.

Someone mentioned that it may be EGR related.
I have the same problem! Read code 1-2-3 which is ECT sensor,replaced it and still have the code and the problem. You found out?
I had a bad ecu and replaced it. Car starts first try every time but i still have a bad idle and hesitation. No CEL yet but I’ll keep posted
If the car ever sat outside in snow or rain, check all of your connections along the edges of the hood especially around the battery and headlights for corrosion, use deoxit, sandpaper, a Dremel. Also grounds, if your ecu went bad it could be because there is a bad ground somewhere and it'll take your new ecu eventually as well. I think the prime issue some have had is oxidation of the terminals of the RSR, copper on alluminum, which is a recipe for corrosion. My resistor pack connector was corroded, confusing either the ecu or the injectors, I cleaned it yesterday gave me a noticeable improvement.
I think the ECU went bad just because it’s an in reliable computer. It wasn’t entirely bad and still somewhat functioned when warm, just issues in the cold. I did clean some grounds in the engine bay and the one near the computer was perfect already.
I plan(ned) on doing an EGR delete soon since i do not believe it to be functioning properly, and i also want to be able to do PCV box maintenance easier later on. I know to do this without causing a CEL I’ll need a non EGR EZK (148?). So I’ll get this.

Now, i just realized this car is also a cold start injector car. Neither of my 240s have a CSI and they are perfectly fine even down below 0 F. So who knows if this thing is leaking or not. I have installed a 951 ecu and have not yet set a CEL even though the 951 is a non cold start injector ecu. I’m assuming that if it was vice versa (CSI ECU with the CSI absent/unplugged) that would cause a CEL?

I know most of you will say it’s pointless to get rid of egr and csi, but I’m only doing it for simplification. I plan on heading to the JY and grabbing a non egr non csi intake manifold tomorrow.

Now to block off the egr at the exhaust manifold.
Anyone have any pictures as to how to block it off at the exhaust without switching manifolds? Special fitting?

Edit: nvm, it’s m18x1.5 just like an o2 sensor. I’ll go pick one up.
Had a similar issue with my 240. Ended up firing the parts cannon (which brought it up to stage 0), but found a few really small holes worn in the intake hose between the MAF and the intake. Once those were fixed that seemed to take care of it.
Had a similar issue with my 240. Ended up firing the parts cannon (which brought it up to stage 0), but found a few really small holes worn in the intake hose between the MAF and the intake. Once those were fixed that seemed to take care of it.

I haven’t fired the parts cannon yet I’m just trying to simplify this car. And I’m also doing a somewhat stage zero
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Internet was wrong, it’s not m18x1.5.

It’s smaller than that. Any ideas? Need to get this plugged up ASAP.

Ah wait a second. The manifold itself is smaller than m18x1.5. The first part of the egr you take off is m18x1.5 but you need female threads. Not an o2 sensor or o2 sensor bung. They’re both male.
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I wasn't going to drive all over town trying to find the corrrect fitting, so I crushed the EGR pipe, heated it, and dropped in some solder. It didn't even leak before the solder but I looked at it as cheap insurance.



And then I found this post AMM air leak that I have no excuse for not catching earlier:oops:

So, egr is all deleted, new intake manifold without egr/csi installed, 93+ fuel rail with schrader valve too. Got a 146 ezk to install to keep the CEL away after getting rid of the egr stuff.