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Diagnostics Tech

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09 Aug 2019 00:34 #32651 by randy61
Diagnostics Tech was created by randy61
I need a diagnostics tech but don't want to end up at a shop who will just throw parts.

Can anyone recommend questions to ask to prequalify a shop for diagnostics?

Any input is appreciated. Thank you.
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09 Aug 2019 09:15 #32658 by Chad
Replied by Chad on topic Diagnostics Tech
Ask them if they can explain Long Term and Short Term Fuel Trim.
What is the 5 volt reference?
How do you do a voltage drop test?

"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."
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09 Aug 2019 09:43 - 09 Aug 2019 09:44 #32660 by Columbus450
Replied by Columbus450 on topic Diagnostics Tech
^^^ nice ones A good tech should be able to explain this ,,a parts changer would not be able too.

I have a few more;

name a few possibilities that could create a lean issue ?

Name a few possibilities that could create a rich issue?

How would you test a suspicious bad ground wire?



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Last edit: 09 Aug 2019 09:44 by Columbus450.

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09 Aug 2019 11:45 - 09 Aug 2019 11:45 #32662 by Noah
Replied by Noah on topic Diagnostics Tech
The shops that I see with talent in that area advertise it.
"Electronic specialist" or "computer diagnostics" on the sign out front are sevices that someone with difficulty in that area probably wouldn't be trying sell.
Not fool proof of course...

"Learn, apply, repeat."
Last edit: 09 Aug 2019 11:45 by Noah.

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09 Aug 2019 12:16 #32663 by Andy_
Replied by Andy_ on topic Diagnostics Tech
Hi Randy.

What is the issue you are having with the vehicle? Maybe if you tell us that we can give you more questions to ask.

I get it a lot where people call and say, "I need a tune-up, can you give me a price?" This is always the perfect opportunity for me to ask why they want a tune up. 99 out of 100 times it's not because they feel like spending money on spark plugs. It's because they have a misfire. When I take the time to tell them there is a lot of reasons for a misfire and explain a few, I usually end up winning them over as customers.

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09 Aug 2019 13:15 #32664 by 9w1911
Replied by 9w1911 on topic Diagnostics Tech
I am looking for one in the denver area unless someone here wants me to Paypal them for their time to help me out.

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09 Aug 2019 13:31 #32666 by Andy_
Replied by Andy_ on topic Diagnostics Tech
What do you need to know, 9w1911? Maybe I can help you for free.

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09 Aug 2019 13:34 #32667 by 9w1911
Replied by 9w1911 on topic Diagnostics Tech
Hey Andy, I have some issues with trouble codes from rodent damage or wire harness damage and am just a beginner when it comes to vehicle electronics. Also, none of the shops in my area have a specialist to fix this and I live hours away from a city. So I am praying I can fix this myself.

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09 Aug 2019 13:39 #32668 by Andy_
Replied by Andy_ on topic Diagnostics Tech
9w1911, why don't you start your own thread about your vehicles issues? I'd be glad to try to talk you through the wiring problems as much as I can.

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09 Aug 2019 13:42 #32669 by 9w1911
Replied by 9w1911 on topic Diagnostics Tech
I did but I think its too much ask or its too many issues:

www.scannerdanner.com/forum/post-your-re...p1721-and-v1001.html

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09 Aug 2019 14:06 #32670 by Andy_
Replied by Andy_ on topic Diagnostics Tech
Ok, I'll look over there and respond.

Just give me some time. Busy with cars at work right now.
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12 Aug 2019 00:02 - 13 Aug 2019 10:13 #32741 by randy61
Replied by randy61 on topic Diagnostics Tech
Thanks all for your suggestions.

Since you asked Andy, and thank you, here is my vehicle and the issue I am having.

1990 Jeep Cherokee renix engine 4.0 stalls misfires. Idles fine when cold and after warmup but upon driving after warmup it will loose RPM after applying the brake it will try to quit if I don't give it gas and sometimes it will stall.

It will restart but runs like crap and missfires badly untill it cools down then idles fine again untill driving then everything starts all over again.

Note: When it is cold after startup if I snap the throttle it will momentarily want to die and run crappy just for an instant until it re-stablizes

Was running great until after I had made several freeway trips, dont normally drive freeway, it then began wanting to stall at stop lights and it continually got worse.

New Parts:
1 year prior to this issue I had replaced the fuel pump (it went out). I replaced the oxygen sensor from a junk yard vehicle and the IAT-idle air stepper motor (because prior to the issue having now it was running goofy. Those components seemed to fix that issue. (Note: I put a writeup as to what it was doing then on this forum, can't seem to find it now. I had gotten a few hundred views but no one offered any advice.

Since this current issue began I replaced these components. (Note: Sensors replaced due to not working properly according to my MT2500)
MAT sensor (was reading high), Oxygen sensor (it wasn't switching), Coolant temp sensor (only because I replaced the thermostat, it was in the block), Thermostat ( was getting inconsistent temp readings), intake exhaust gasket ( I had an intake/exhaust leak), cleaned throttle body although it was fairly clean, new vacuum lines (they were old and brittle), starter relay (because I got parts from a junk yard vehicle and it had a new one) and fuel regulator (just because I put in new injectors. Original ones are known to leaking and didn't want to take a chance of a future fire)

Spark plugs , wires rotor and cap have 7,000 miles on them. No water in oil and no oil in water. Compression 130-135. Cat temp 75 degrees cooler in rear than the front.

I have a DRB II that diagnose Chrysler vehicles before OBD 1 came out. The computer doesn't store codes but the DRB II gives fault codes. The Fault code I got was Fault 1052 which is MAP sensor out of limits.

In the fault code book it has a series of flow charts that gives DMM test and the chart ends up taking me to a D3 Sensor Short to Ground.

I tore the harness apart following all three MAP sensor wires and found nothing.

Thanks everyone.

I watch Paul's videos a lot and am blown away by his expertise in diagnosing vehicles. I only wish that I wasn't so far away as I would love to watch him work on it.

Note: I watched Paul's video on Hall Effect cam/crank sensor operation testing part 1 and 2. Almost feel like my issue lies there somewhere although have not done testing due to not knowing how.

Monday 12th will include link to video of engine running on MT2500 so you can see the LTFT and STFT- doesn't look correct and don't know what it should be for this vehicle.
Last edit: 13 Aug 2019 10:13 by randy61.

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12 Aug 2019 01:53 - 14 Aug 2019 23:12 #32742 by randy61
Replied by randy61 on topic Diagnostics Tech

randy61 wrote: Thanks all for your suggestions.

Since you asked Andy, and thank you, here is my vehicle and the issue I am having.

1990 Jeep Cherokee renix engine 4.0 stalls misfires. Idles fine when cold and after warmup but upon driving after warmup it will loose RPM after applying the brake it will try to quit if I don't give it gas and sometimes it will stall.

It will restart but runs like crap and missfires badly untill it cools down then idles fine again untill driving then everything starts all over again.

Note: When it is cold after startup if I snap the throttle it will momentarily want to die and run crappy just for an instant until it re-stablizes

Was running great until after I had made several freeway trips, dont normally drive freeway, it then began wanting to stall at stop lights and it continually got worse.

New Parts:
1 year prior to this issue I had replaced the fuel pump (it went out). Then I replaced the coil because it went bad. I replaced the oxygen sensor from a junk yard vehicle and the IAT-idle air stepper motor (because prior to the issue having now it was running goofy. Those components seemed to fix that issue. (Note: I put a writeup as to what it was doing then on this forum, can't seem to find it now. I had gotten a few hundred views but no one offered any advice.

Since this current issue began I replaced these components. (Note: Sensors replaced due to not working properly according to my MT2500)
MAT sensor (was reading high), Oxygen sensor (it wasn't switching), Coolant temp sensor (only because I replaced the thermostat, it was in the block), Thermostat ( was getting inconsistent temp readings), intake exhaust gasket ( I had an intake/exhaust leak), cleaned throttle body although it was fairly clean, new vacuum lines (they were old and brittle), EGR valve (because I did some grinding for suspension mods and burned the EGR components), starter relay (because I got parts from a junk yard vehicle and it had a new one) and fuel regulator (just because I put in new injectors. Original ones are known to leaking and didn't want to take a chance of a future fire)

Also since the issues began I had replaced the ignition switch, not the ignition cylinder. Replaced because there had been a short and melted the coating on the wire that connects the tail lights, heater blower, turn signals, etc. And the ignition switch itself had melted somewhat where the wire was attached to its connection. The ignition key was binding upon turning the key to off position so I think it had shorted due to the catching. Strange thing was the connection itself seemed to be okay, just had some melting.

Spark plugs , wires rotor and cap have 7,000 miles on them. No water in oil and no oil in water. Compression 130-135. Cat temp 75 degrees cooler in rear than the front.

I have a DRB II that diagnose Chrysler vehicles before OBD 1 came out. The renix computer doesn't store codes but the DRB II gives fault codes. The Fault code I got was Fault 1052 which is MAP sensor out of limits. In the fault code book it has a series of flow charts for the MAP that gives DMM test and the chart ends up taking me to a Short to Ground. But the MAP seems to be working according to my MT2500.

I tore the harness apart following all three MAP sensor wires and found nothing.

Thanks everyone.

I watch Paul's videos a lot and am blown away by his expertise in diagnosing vehicles. I only wish that I wasn't so far away as I would love to watch him work on it.

Note: I watched Paul's video on Hall Effect cam/crank sensor operation testing part 1 and 2. Almost feel like my issue lies there somewhere. Not sure if that applies to my vehicle

Here's a video of engine running on MT2500 so you can see the LTFT and STFT- doesn't look correct and don't know what it should be for this vehicle.

Sorry for the glare, didn't realize it was there.
LTFT is 128 128 is 0 correct?

Last edit: 14 Aug 2019 23:12 by randy61.

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12 Aug 2019 08:37 #32745 by Andy_
Replied by Andy_ on topic Diagnostics Tech
Hi Randy, that's quite a list of things you've done. Not that new parts means good parts, but if the symptoms are still the same, then it's probably something completely different.

One thing that comes to mind is the EGR. Have you checked if it is sticking open? I can't think off the top of my head how this EGR is, but EGR's sticking open and causing stalling at slower speeds used to be pretty common.

A really quick way to eliminate the EGR is to cut a piece of an aluminum pop can to block the EGR passages, then bolt the EGR back down. This is temporary.

EGR sticking open can be hard to see in fuel trims. EGR gas is inert, it's not going to really show lean or rich, but it will cause what amounts to a big vacuum leak when it's stuck open, which could explain the MAP code.

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12 Aug 2019 13:32 - 14 Aug 2019 23:10 #32750 by randy61
Replied by randy61 on topic Diagnostics Tech

Andy_ wrote: Hi Randy, that's quite a list of things you've done. Not that new parts means good parts, but if the symptoms are still the same, then it's probably something completely different.

One thing that comes to mind is the EGR. Have you checked if it is sticking open? I can't think off the top of my head how this EGR is, but EGR's sticking open and causing stalling at slower speeds used to be pretty common.

A really quick way to eliminate the EGR is to cut a piece of an aluminum pop can to block the EGR passages, then bolt the EGR back down. This is temporary.

EGR sticking open can be hard to see in fuel trims. EGR gas is inert, it's not going to really show lean or rich, but it will cause what amounts to a big vacuum leak when it's stuck open, which could explain the MAP code.


I forgot to add it to my list of replacement parts.
I had cut off the upper control arm brackets in order to install a longarm suspension. The sparks from my cutting disc had melted the EGR hose along with that black disc that attaches to the EGR. So the original EGR was not functioning when it was running fine before my current issue. So we can rule out the EGR as suspect. Thought I would mention that the EGR is controlled by a electric solenoid.
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Last edit: 14 Aug 2019 23:10 by randy61.

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14 Aug 2019 07:29 #32805 by Andy_
Replied by Andy_ on topic Diagnostics Tech
I went back and read your original post. You said it will idle fine, but after you drive it and press on the brakes it will idle rough and want to die. Is this correct?

If so, when the problem is happening, try clamping off the hose to the brake booster. I sort of doubt this is the problem, but it's worth a shot. Just don't drive it with the brake booster hose clamped.
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