D-jet trouble shooting

Drew20

Drew20

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I have a rich running issue, not terrible, but my coupe won't idle well in traffic (on a stop at the lights the idle starts off lumpy, gets lumpier and eventually stalls). I did a CO check but couldn't get the CO level in spec, even with the mixture screw (on the ECU) turned to max lean.
I've now checked the system from the ECU plug wiring and have some ideas, and wanted to get some expert input before I start taking things apart.
Looks to be no problem with the wiring: grounds and power all check out. As do the injectors (electrically, not checked flow or pattern or leakage), the fuel pump and the TPS. The MAP sensor also checks out fine (electrically at least)
I came up with some options to test for:
- Leak on MAP sensor, or the hose to it (reads low manifold vac and so over fuels)
- Leaky injectors (over fuelling)
- Blocked coolant lines near coolant temp sensor
- Coolant temp sender fault
- Air temp sender fault
- Fuel pressure high , i.e. fault with FPR

With the engine running I took some temp readings with a non contact thermometer, this indicated that the engine was running at 75C (167F) but that the coolant temp sender was only 50-55C (130F). So I think I might have a blocked coolant line around the coolant temp sender. I found a good thread about on this (I'll post a link when I find it again) and it sounds like a weekend's work to strip this down, clean and rebuild.
I'll test out the 'stat and the temp sender also.
Question: the thermo time switch, i know this controls the cold start injector, but how does it work? I read that it is also electrically heated, but there are only two wires going to it

I'll update this post, and move on to testing fuel pressure etc.
 
Last edited:
3 quick ideas.
Disconnect the cold start valve.
Pinch off the cold start fuel hose
Test for vacuum the MAP sensor.

And always test for good spark when you think you have a fuel problem!
 
Thanks Don, I took your points generally and did an overall engine health check this evening, always good to have a good base before taking things to pieces. Spark was good on all plugs/cylinders, compression is good I think (190psi across the board except for no.5 which was 200psi)
Plugs don't look too fouled. Certainly some carbon build up in the head, but plug tips are clear, see photos.
I also checked fuel pressure, which was 28psi running, dropping to 25psi after 2 mins, 23psi after 5 mins, 21psi after 10 mins and 19psi after 20 mins. Which I think is ok? (Test done without CSV in though, need to look at that tomorrow, check no leaks and operating ok)

Without any tool to pull a vacuum, what's the best way to test the MAP? I do have a manifold pressure gauge which goes down to -30 inches of mercury!! Yes, it's a proper antique! But nothing to pull and hold a vac. Something I need to buy maybe
 

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In the states that is 25.00!

Those plugs are black and sooty,
Test for vacuum/ruptured MPS
 
Done a leak test on the MAP, it held vac pretty well. I put 16" Hg on it and that fell at 1" Hg per minute. So think the diaphragm is ok, not sure the aneroid cells can be tested?

Thanks Don
 
getting to it!! I know, I'm slow, work's been busy. :)

will take the car for a run tonight with CSV disconnected, and I plan to fit a test light on the power for the CSV so I can see how the ECU is controlling the CSV (I think CSV should operate only quite briefly after a cold start). The CSV fuel hose currently has a pressure gauge attached to it, will just leave it there, so CSV disconnected on fuel and power. Aux air will be left connected up.

here are some posts I found re the coolant "manifold" underneath the intake runners, mainly for my reference, so I can find them again!
https://e9coupe.com/forum/threads/h...sing-unwanted-cruise-control.3099/#post-17028
https://e9coupe.com/forum/threads/blow-by-or-fuel.12996/#post-86059
https://e9coupe.com/forum/threads/csi-fuel-injection-controller.14256/#post-96137

I'm seeing a big temp drop across this, so it needs to come off for a de-gunk

thanks again
 
I have the replacement coolant blocks if you need one.
 
Right, updates on stuff

CSV looks to be fine, with the CSV connected up the residual fuel pressure dropped as slowly as before, so no evidence of a leak. It's only powered for a short period after cold start. Think it's ok

Manifold vac sits around 15-16 "Hg. This seems a bit low to me. Any idea what it should be? The car idles at 1100rpm. I tried to bring that down to 950, but with idle screw tightened the idle got even worse, engine almost stalling.

I've got a vid of the engine running, will try to upload it
 
Drew - not so long ago i had my engine rebuilt and balanced by Fritz's Bits in Somerset.

I had previously experienced some difficulty in getting much adjustment from the ECU, so got them to modify my MPS.

They describe the procedure on their website www.fritzbits.co.uk as follows


The system uses an inlet manifold vacuum unit to measure engine load, which then transmit’s the information to the ECU (often positioned underneath the back seats) which operates the injectors and fuels accordingly. The main issue we have noted here at Fritz, is that the vacuum unit seems to slip out of adjustment over time, resulting in a rich mixture throughout the rev range. Looking at the unit, there appears to be no real method of adjustment available – however it is possible. At one end of the unit, there is an aluminium blanking plug, with a groove cut into it, much like the head of a basic wood screw.


With a drill and 6mm drill bit, carefully drill through the plug, which is about 8mm thick. Once through the aluminium, a brass adjustment screw will be revealed. Adjust the screw, with the car running on a gas analysers, and you should be able to alter the CO content of the emissions back to sensible levels. Remember also to check the CO levels at higher revs too, usually setting up the mixture at a static 3000rpm gives a good indication that the fuelling will be right across the rev range.

They did a great job on my engine - so would be happy to recommend them
 
When you reduce the idle by screwing in the idle screw you reduce the air going into the engine which gives you a richer mixture. Not good.
The MPS is just a 3 position switch operated by vacuum. I believe idle is at 18”hg vacuum meaning your computer thinks you are at cruise. Also not good.
Cap the vacuum nipple at the plenum and Try your hand held vacuum pump to bring vacuum to 19”hg.
Your idle may improve.
May be time to hunt down vacuum leaks.
I would start with the oil dip stick.
Install an o-ring in the groove at the top and make sure it is tight in the tube.
 
Djet is super simple and dependable... but everything needs to be tested and adjust in an exact sequence. Start with making sure you have good vacuum. It doesn't have a lot of control range and it was designed and set to work on a fresh engine with optimum vacuum. A worn engine (or leaky) with low idle vacuum will want to run rich with this system as Don mention above.
 
sfdon said:
When you reduce the idle by screwing in the idle screw you reduce the air going into the engine which gives you a richer mixture.
Click to expand...
Thanks again for your input, I'm learning a load.
I tightened the idle screw to see what impact it had on vac... no impact, just a rubbisher idle

How does the MPS/MAP interact with the TPS? I'd read the ECU got its idle signal from the TPS. I'll get onto looking for vac leaks. I built a piece of kit to do this once, basically a very sensitive manometer. I'll see if I still have it....

Gut feel is the engine is pretty good, compression is good. I have yet to do a leakage test, but I don't think that's going to tell me much anyway. I'll clean out the coolant "manifold" to get rid of that issue, and hunt down vac leaks. Is spraying carb cleaner around the way to go here?

Thanks both, much appreciated
 
Christopher said:
Drew - not so long ago i had my engine rebuilt and balanced by Fritz's Bits in Somerset.

I had previously experienced some difficulty in getting much adjustment from the ECU, so got them to modify my MPS.

They did a great job on my engine - so would be happy to recommend them
Click to expand...

Thanks for that, I'll see if it comes to that. My engine was rebuilt last in the late 80s, but has only done 30k since. And it feels tight enough, just lumpy. My own experience of FB is mixed
 
Carb cleaner with a “straw”
Go slow and thorough.
TPS acts as a fuel enrichment/acceleration device.
How much throttle and fast throttle.
I’m pretty sure 18”hg is standard.
Use your vacuum pump to go to 20”
Check idle.
Have access to a know good unit (MPS)??
 
Thanks Don, got coolant system in pieces to improve the flow past the temp sensors. I don't have access to another sensor, but will check for leaks once it's all together again

The small coolant "manifold" is connected to the large external return pipe and to something underneath the throttle body, and then this connects to a port on the block (I think, can't quite see)
Which way does coolant flow through this circuit?
And what is the thing underneath the throttle body, and why does it need coolant flow?
 
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