Diagnosing an intermittent D-Jet problem..., Hesitation under load at 2800 RPM but only when cold... Options

[MikeInMunich]

Is this a clue? Driving this evening, with a faulty AAR I'm testing, while the engine was cool, the bocking was NASTY in traffic, in 1st and especially 2nd gear aroun 2400 RPM. When the engine is then warm, it runs much better, and the issue discussed here is reduced to a slight hesitation, not "bocking".

Does this tell us anything?

Thanks,

M.i.M.

[MikeInMunich]

First suspect would be the tps adjustment. Usually causes a miss at steady cruise. Caused by a worn circuit board inside unit. Sometimes they are just dirty and need cleaning.

I bought a new TPS plate from Dave Sprinkle, a member here. Very disappointingly, this did not relieve the symptom! (IMG:style_emoticons/default/sad.gif)

[pbanders]

Still sounds like an intermittent contact issue.

Sounds like you've ruled the CHT out. FWIW, I contort the $#%@ out of myself when checking the CHT connection to get into the engine compartment so I can make 100% sure that the female spade connector from the CHT is actually on the male connector inside the plastic insulator from the wiring harness - it is <extremely easy> to think it's on when it's actually jammed between the plastic and the male connector. Might be worth one more check.

If it's not the CHT, I'd expand the search. First thing I'd do is find a glass-smooth section of road. Drive on it in the RPM range where you have the problem. Is the bucking still there? Now, drive on a rough section of road. Worse? An intermittent contact is suspect. If not, it's probably something else.

If you think it's an intermittent contact, check the relay board connectors. Check the underside of the relay board to make sure the epoxy coating is still in place and there's no contact corrosion. Swap in new relays on the board for the power supply (74) and the fuel pump (75). Check the ECU connector. Check all of your ignition connectors - BTW, what kind of ignition setup do you have? Check your plug wires and make sure they're in good shapee and aren't loose at the plug connectors. Check your ignition switch. Check the fuel pump connector at the pump (I had a crappy one that caused problems years ago). Check the FI grounds in the back of the engine.

What kind of shape is your wiring harness in? Most OEM harnesses I've seen have gotten pretty bad over the years. Some of the best money I ever spent on my car was buying a new harness from Jeff Bowlsby.

Other than intermittent contact, look further at your ignition. Try swapping in a new distributor cap and rotor, and new plug wires. Try swapping your coil. Make sure that the ground strap inside of the distributor for the points plate isn't broken or frayed. Check your distributor shaft wobble and make sure it hasn't become excessive. Put a timing light on the car and see if your timing mark is stable. If you have aftermarket ignition like a Pertronix, Crane, or MSD, try swapping back to conventional points, condenser, and coil to see if that's the issue.

As far as other components in the FI system, the pressure sensor is another possibility. Check my web page (sig below) for photos of the internals. It could be that at a specific part-load level it's hanging up, possibly due to contamination or to a broken component. Only way to really check this is to swap in a known good or NOS unit. Yes, the MPS is really expensive, but if you have a D-Jet car I strongly suggest that you have at least one or more spares on hand, just part of the price of owning a 40+ year-old car.

I'm sure others will have some additional suggestions. You need to be methodical and eliminate possibilities one at a time. I'm sure our various ASE certified mechanics here in the forum will tell you that's how they're trained to do it. Good luck, you will find the problem and fix it.

[mepstein]

Worth double-checking all the electrical connections.

--DD

(IMG:style_emoticons/default/agree.gif)

[StratPlayer]

I was having that problem,,,hesitation,,,,, I just replaced the TPS board and the problem is solved.... No more hesitation at all.

[pbanders]

More ignition checks: Verify that your vacuum advance on the distributor is working. If it's not connected, make sure it's plugged. If it is connected, pull off the distributor cap, take a hand vacuum pump, and pull a vacuum on the port. You should see the points plate move until the advance reaches the stop. Also check for free operation of the points plate, they're often super-grimy/gritty and won't slide against each other.

[MikeInMunich]

I was having that problem,,,hesitation,,,,, I just replaced the TPS board and the problem is solved.... No more hesitation at all.

I WISH that was my result!!

[MikeInMunich]

Hi Brad, thanks for all your advice. I will be looking into these potential sources of the problem. Oh how sweet it would have been to have this issue disappear after changing the TPS plate! Is there anything perhaps that the mechanic could have done wrong while changing that whichwe should check on?

The distributor is electronic. Is there still a vacuum advance on it? (IMG:style_emoticons/default/confused24.gif)

Thanks again for your time and assistance! (IMG:style_emoticons/default/beerchug.gif)

M.i.M.

[socal1200r]

Hi Brad, thanks for all your advice. I will be looking into these potential sources of the problem. Oh how sweet it would have been to have this issue disappear after changing the TPS plate! Is there anything perhaps that the mechanic could have done wrong while changing that whichwe should check on?

I'm hoping he "set" the TPS module with an ohm meter? It's not just a matter of installing a new circuit board, the "blades" have to be set to zero across the circuit board with an ohm meter, then one mark more to be properly adjusted. There's a video either on here on or YouTube that shows how to do this properly.

[Matty900]

Gary Cotton did 2 great videos on this.
Part 1
https://youtu.be/6x4TBwXlJu4

Part 2
https://youtu.be/J_yzQB7X4Uk

Check them out.
Hope that helps
(IMG:style_emoticons/default/driving.gif)

[MikeInMunich]

Thanks again guys! Huge help!!

Have a great weekend!

M.i.M.

[MikeInMunich]

Update, edited!

Unfortunately I don't trust the guy who installed the replacement TPS plate I got from Dave Sprinkle. The hesitation still persists. Also, when the engine is cold and I turn the knob on the ECU there is NO CHANGE to the idle speed at all. According to Mr. Bowlsby's posting about the system, this is an indication that the TPS isn't functioning or isn't adjusted correctly.

As mentioned above, I measured the resistance at the CHTS connector to be > 2000 at ambient temperature (summer, ca. 25 C) and under 300 Ohms when the engine was warm.

When the hesitation is happening, under load at about 2800 RPM it doesn't persist when I take my foot off the pedal.

Another hint is that it ceases to occur all but 100% after the engine is FULLY warm.

What do you think?

Thanks,

M.i.M.

[MikeInMunich]

(IMG:style_emoticons/default/icon_bump.gif) Seeking expert's feedback on D Jet Problem diagnosis...

I had posted that the problem seemed to have been solved. I was unfortunately mistaken and have edited my update in the comment above this one. Expert fedback would be much appreciated!

Thanks!

[914_teener]

So did you do everything Brad noted?

I would suggest methodical checklist approach..

[MikeInMunich]

So did you do everything Brad noted?

I would suggest methodical checklist approach..

That was awesome of Brad to take the time to type up such a long list, and yes, most, though not all of those things were checked. It was quite exaustive. I'm pretty sure that it's either the TPS or a bad connection, likely at the CHTS.

Just wondering right now if anyone has a clue as to why the symptom would stop when the car is fully warm.

Thanks,

M.i.M.

[914_teener]

So did you do everything Brad noted?

I would suggest methodical checklist approach..

That was awesome of Brad to take the time to type up such a long list, and yes, most, though not all of those things were checked. It was quite exaustive. I'm pretty sure that it's either the TPS or a bad connection, likely at the CHTS.

Just wondering right now if anyone has a clue as to why the symptom would stop when the car is fully warm.

Thanks,

M.i.M.

If you have checked everything he posted........it should run fine.


Two things....if it is an electeical problem the symptom may be stopping because the connection heats up and the intermittent connection is no longer intermitent. Meaning copper commections have high coefficents of expansion and make better contact.

If it is fuel then....same thing....fuel atomizes better in warm air.

Have you tried upping the fuel pressure to 32 psi instead of 29?

[MikeInMunich]

[/quote]
If you have checked everything he posted........it should run fine.

Two things....if it is an electeical problem the symptom may be stopping because the connection heats up and the intermittent connection is no longer intermitent. Meaning copper commections have high coefficents of expansion and make better contact.

If it is fuel then....same thing....fuel atomizes better in warm air.

Have you tried upping the fuel pressure to 32 psi instead of 29?
[/quote]

I have not tried upping the fuel pressure. Thanks for the tip regarding heat and the possibility of the problem being a bad connection. If it's a bad connection I'm presuming it's most likely the CHTS connection to the harness. Not sure how warm that would get though. (IMG:style_emoticons/default/confused24.gif)

[MikeInMunich]

Update: Disconnected the TPS today and tested...Problem was still there. So this rules out the TPS, correct?

Then I checked the CHTS connection. I'm pretty darn sure it must be good. As previously mentioned, the Ohm test on that was well within parameters, over 2000 at ambient temp and under 300 after engine running until warm.

So I guess I will now have to go through Brad's entire list after all.

M.

[Bob L.]

Update, edited!

... Also, when the engine is cold and I turn the knob on the ECU there is NO CHANGE to the idle speed at all. According to Mr. Bowlsby's posting about the system, this is an indication that the TPS isn't functioning or isn't adjusted correctly.


M.i.M.

As far as I know, the knob on the ECU adjusts the fuel mixture not idle speed.

Mr. Bowlsby probably knows something about this that I don't.

[JeffBowlsby]

The ECU knob is a fine adjustment to fuel mixture AT idle. It has no effect off idle.