Jump to content

trying to diagnose problem - engine is sloppy but only under load


icetitan

Recommended Posts

Hi , I had to go to a new mechanic to help my '78 280z pass smog in early June. They tweaked the AFM and took it to 'their guy' for me and got it to pass. BUT , the car has not driven the same ever since. The car would start up in the mornings (warm weather , Los Angeles) and run a bit sloppy (misfiring , stumbling , but no back-firing though ) . Then warm up and be fine for the most part , but I felt like I had a little bit less power. I took it back to them a week later and asked them to look at it and left it with them while I was out of town for a few days. They said there was nothing wrong with it. This is because they were just starting it in the morning and listening to the engine , but not driving it. The engine sounds fine and not sloppy when in neutral, even when I gas the throttle. The sloppiness only occurs when I shift into a gear/under load.  I insisted he drive it around with me in the passenger seat. He did and finally understood what I meant. Then he adjusted the AFM again and again checked the timing , thinking that would fix it. It did not. I took the car home. Over the next couple days the sloppiness got even worse , but it is intermittent (sometimes when I gas it, it snaps into order and feels like the old car I use to know, but then the sloppiness comes back momentarily). It started sounding like a motorcycle. I called him back. He then said is was the fuel injector wire connections that were bad and corroded , and I should come back and have him do them (or do them myself). I got all new fuel injectors last year, so those can't be the problem. I also listened to each fuel injector with a screwdriver and they were all ticking and working, so the electrical connections are not the culprit , unless they somehow cut-out as I'm driving rather than just parked with the engine running. I just replaced my spark plug wires with new nice ones.  This seemed to help a little bit. I still need new spark plugs but I don't think that is my issue ultimately. I replaced the fuel pressure regulator last year, and the fuel pump about 2 years ago. Any ideas on what the heck could be the problem here ? Could this be a vacuum leak ? Any input would be appreciated  - thank you

Link to comment
  • Replies 18
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

New NGK for that car, cheap and you can do this yourself. May fix it, won't hurt if it isn't. Also you can 'read' the plugs as they are a window to what is going on inside the combustion chamber. Take for a long drive first to remove accumulated carbon.

 

Dry black and powdery...... severely running rich

Porcelain shiny white......... severely lean mixture

Tan or light brown.............. about right

 

Loose isn't really the right word for how badly it runs. Balky, hesitant, rough, stuttering.... mean more. Hard to imagine a loose running engine.

  • Like 1
Link to comment

Assuming the car ran perfectly before they "tuned it", the only things they could have adjusted are they AFM and the timing. What else is there? There is probably an air bypass on the throttle body. They could have adjusted that too.

 

Do you know what your timing was set at before?

 

Do you know how to adjust the AFM? Did they remove the cover to do that?

 

There has to be someone in the LA area that is good at tuning old Datsuns. There's Classic Datsun Motorsports in Vista. He may be willing to help.

Link to comment

A retarded ignition would only cause a loss of power. It will run just fine but lack efficiency. It would not explain running rough, sputtering, stuttering, back firing through intake or exhaust or balking ..... assuming 'loosness' means one of these.

 

If anything running too much timing for the fuel octane rate being used will cause pinging and acceleration under load to be rough, sputtering, stuttering, back firing through intake or exhaust or balking ..... assuming 'loosness' means one of these.

 

 

 

I imagine the mixture is out of whack. Check the old plugs when you put new NGK plugs in there. And do it yourself so you have first hand knowledge about this you can relay here. Your new mechanic is a technician, what today passes for a mechanic. They know nothing about adjusting and tuning these old cars and only about replacing module parts that fail in new cars.

  • Like 1
Link to comment

Thank you all. I'll investigate. Mike , I'll inspect plugs and do them and see what happens. When I say 'sloppy' I mean , sounds like timing is off , or misfiring (no back fireing).. sounds like a muscle car with rough idol , not like the usual L28 sound. thanks everyone

Link to comment

The '78 will have an EI distributor. No points but cap and rotor could be carbon tracked, cracked or worn out. As load is placed on an engine it takes more voltage to jump the plug gap. Electricity always seeks out and finds the easiest path to ground. Your ignition system should be in as good a shape that the plug gap IS the easiest path to ground.

  • Like 1
Link to comment

UPDATE --- OK.. Good News.   I replaced the spark plugs and wires and it's running like new again. Simple fix, but a WORLD of difference. I'm still surprised that the mechanic didn't go for this right off the bat and instead suggested other issues , and even tried to say it was 'fine' after having it for 10+ days sitting at the shop. He's a mechanic, he should know these types of things right away I'd think. Now I know , thanks to all of you kind folks. Thanks everyone.

  • Like 2
Link to comment

Because plugs and wires are relatively cheap and ez for anyone to get. He isnt likely able to get someone to pay him 3-4 times the money to replace them. Always remember,ck the stupid shit first and dont overthink it. Most problem r fairly ez. And we'll get u thru the ones that aint.

  • Like 1
Link to comment

I can see both sides.

 

Throwing parts at a problem to find that they were the problem = cold beer and pat on the back. Yaaaaayyyyy.

 

Throwing parts at a problem to find that they were not the problem = having to explain to customers  why the bill was high or eating the cost of the parts. Booooo.

 

Diagnosing plugs is pretty easy so he should have caught that, if they were in fact dirty.

 

Diagnosing plug wires can be done with a multi-meter, but this takes time. Time = money.

  • Like 1
Link to comment

Good point Stoffregen...

 

Also , I was still curious...  is the TPS the Throttle Position Switch or Throttle Position Sensor , or are they the same thing ?

 

As I stated earlier, we are assuming the car ran good before the smog tuning.

 

It is a 35 year old car though and things are probably on their way out. A worn TPS or fuel pump that's starting to fail could give you the symptoms you are experiencing.

  • Like 1
Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.